graphile-migrate

Opinionated SQL-powered productive roll-forward migration tool for PostgreSQL.

Crowd-funded open-source software

To help us develop this software sustainably under the MIT license, we ask all individuals and businesses that use it to help support its ongoing maintenance and development via sponsorship.

Click here to find out more about sponsors and sponsorship.

And please give some love to our featured sponsors 🤩:

Chad Furman *

Storyscript *

Postlight *

Surge.io *

* Sponsors the entire Graphile suite

Why?

• fast iteration speed — save a file and database is updated in milliseconds
• roll-forward only — maintaining rollbacks is a chore, and in 10 years of API development I've never ran one in production
• familiar — no custom DSL to learn, just use PostgreSQL syntax
• fully functional — sending SQL commands directly to PostgreSQL means you can use all of PostgreSQL's features
• complements PostGraphile — works with any application, but PostGraphile's watch mode means that the GraphQL schema is instantly regenerated (without server restart) whenever the database changes

Opinions

• Local iteration should be easy and fast
• Migrating should be fast
• Once deployed, databases should be identical (including subtleties such as column order)
• Migration software should not be tied to a particular application stack
• Migrations should be written in SQL
• Roll-forward only (production issues should be fixed via additional migrations, development can iterate current migration)
• Once a migration is signed off (deployable) it should never be edited
• Use PostgreSQL ;)
• Development databases are cheap; can run multiple
• Resetting development database is acceptable if absolutely necessary
• Production databases are critical - NEVER RESET
• Migrating data (as well as DDL) is acceptable, but should be kept to fast operations (or trigger a background job)
• Migrations should automatically be wrapped in transactions by default
• Migrations that require execution outside of a transaction (e.g. to enable augmenting non-DDL-safe things, such as ENUMs in PostgreSQL) should be explicitly marked
• Migrations should not pollute PostgreSQL global settings (e.g. use SET LOCAL rather than SET)
• Roles should be managed outside of migrations (since they can be shared between databases)
• Certain schemas are managed by other tools and should not be interfered with; e.g. graphile_worker

Setup

In development, graphile-migrate uses two databases: the main database and a "shadow" database. The "shadow" database is used internally by graphile-migrate to test the consistency of the migrations and perform various other tasks.

In production, most users only run graphile-migrate migrate which operates solely on the main database - there is no need for a shadow database in production.

All members of your team should run the same PostgreSQL version to ensure that the shadow dump matches for everyone (one way of achieving this is through Docker, but that isn't required).

We recommend dumping your database schema with pg_dump after migrations are completed; you can see an example of this in Graphile Starter. Tracking this file in git will allow you to easily see the changes that different migrations are making, so you can be sure you're making the changes you intend to. We recommend that you dump the shadow database as it will be unaffected by the iteration you've been applying to your development database (which may have come out of sync - see 'Drift' below).

Getting started

Create your database role (if desired), database and shadow database:

createuser --pwprompt appuser
createdb myapp --owner=appuser
createdb myapp_shadow --owner=appuser

Export your database URL, shadow database URL, and a "root" database URL which should be a superuser account connection to any other database (most PostgreSQL servers have a default database called postgres which is a good choice for this).

export DATABASE_URL="postgres://appuser:password@localhost/myapp"
export SHADOW_DATABASE_URL="postgres://appuser:password@localhost/myapp_shadow"

export ROOT_DATABASE_URL="postgres://postgres:postgres@localhost/postgres"

Your database URL is needed for most Graphile Migrate commands. The shadow database URL is needed for the development-only commands commit, uncommit and reset. The root database URL is needed to drop and recreate databases, i.e. for the reset command and for commands that call it (commit and uncommit, which reset the shadow database).

NOTE: you should not need the shadow database URL or root database URL in production (you only need the graphile-migrate migrate command in production) unless you have actions that need them.

Then run:

graphile-migrate init

At this point you should be ready to use Graphile Migrate. You may want to store these environmental variables to a file so you can easily source them (with the . command in bash, for example) in future:

. ./.env
graphile-migrate watch

Usage

Committed and current migrations

New migrations are composed within "the current migration". You will see this term used a lot. By default this is in the migrations/current.sql file, but if you like you may delete that file and instead create a migrations/current/ folder into which you may place numbered SQL files which together comprise "the current migration".

The current migration should be idempotent (this is your responsibility, see "Idempotency" below); i.e. it should be able to be ran multiple times and have the same result. This is critical for graphile-migrate watch, which is one of the main selling points of the project.

graphile-migrate

graphile-migrate <command>

Commands:
  graphile-migrate init            Initializes a graphile-migrate project by
                                   creating a `.gmrc` file and `migrations`
                                   folder.
  graphile-migrate migrate         Runs any un-executed committed migrations.
                                   Does NOT run the current migration. For use
                                   in production and development.
  graphile-migrate watch           Runs any un-executed committed migrations and
                                   then runs and watches the current migration,
                                   re-running it on any change. For development.
  graphile-migrate commit          Commits the current migration into the
                                   `committed/` folder, resetting the current
                                   migration. Resets the shadow database.
  graphile-migrate uncommit        This command is useful in development if you
                                   need to modify your latest commit before you
                                   push/merge it, or if other DB commits have
                                   been made by other developers and you need to
                                   'rebase' your migration onto theirs. Moves
                                   the latest commit out of the committed
                                   migrations folder and back to the current
                                   migration (assuming the current migration is
                                   empty-ish). Removes the migration tracking
                                   entry from ONLY the local database. Do not
                                   use after other databases have executed this
                                   committed migration otherwise they will fall
                                   out of sync. Assuming nothing else has
                                   changed, `graphile-migrate uncommit &&
                                   graphile-migrate commit` should result in the
                                   exact same hash. Development only, and liable
                                   to cause conflicts with other developers - be
                                   careful.
  graphile-migrate status          Exits with a bitmap status code indicating
                                   statuses:

                                   - 1 if there are committed migrations that
                                   have not been executed yet (requires DB
                                   connection)
                                   - 2 if the current migration is non-empty
                                   (ignoring comments)

                                   If both of the above are true then the output
                                   status will be 3 (1+2). If neither
                                   are true, exit status will be 0 (success).
                                   Additional messages may also be output.
  graphile-migrate reset           Drops and re-creates the database, re-running
                                   all committed migrations from the start.
                                   **HIGHLY DESTRUCTIVE**.
  graphile-migrate compile [file]  Compiles a SQL file, inserting all the
                                   placeholders and returning the result to
                                   STDOUT
  graphile-migrate run [file]      Compiles a SQL file, inserting all the
                                   placeholders, and then runs it against the
                                   database. Useful for seeding. If called from
                                   an action will automatically run against the
                                   same database (via GM_DBURL envvar) unless
                                   --shadow or --rootDatabase are supplied.
  graphile-migrate completion      Generate shell completion script.

Options:
  --help        Show help                                              [boolean]
  --config, -c  Optional path to gmrc file        [string] [default: .gmrc[.js]]

You are running graphile-migrate v1.2.0.

graphile-migrate init

graphile-migrate init

Initializes a graphile-migrate project by creating a `.gmrc` file and
`migrations` folder.

Options:
  --help        Show help                                              [boolean]
  --config, -c  Optional path to gmrc file        [string] [default: .gmrc[.js]]
  --folder      Use a folder rather than a file for the current migration.
                                                      [boolean] [default: false]

graphile-migrate migrate

graphile-migrate migrate

Runs any un-executed committed migrations. Does NOT run the current migration.
For use in production and development.

Options:
  --help          Show help                                            [boolean]
  --config, -c    Optional path to gmrc file      [string] [default: .gmrc[.js]]
  --shadow        Apply migrations to the shadow DB (for development).
                                                      [boolean] [default: false]
  --forceActions  Run beforeAllMigrations and afterAllMigrations actions even if
                  no migration was necessary.         [boolean] [default: false]

graphile-migrate watch

graphile-migrate watch

Runs any un-executed committed migrations and then runs and watches the current
migration, re-running it on any change. For development.

Options:
  --help        Show help                                              [boolean]
  --config, -c  Optional path to gmrc file        [string] [default: .gmrc[.js]]
  --once        Runs the current migration and then exits.
                                                      [boolean] [default: false]
  --shadow      Applies changes to shadow DB.         [boolean] [default: false]

graphile-migrate commit

graphile-migrate commit

Commits the current migration into the `committed/` folder, resetting the
current migration. Resets the shadow database.

Options:
  --help         Show help                                             [boolean]
  --config, -c   Optional path to gmrc file       [string] [default: .gmrc[.js]]
  --message, -m  Optional commit message to label migration, must not contain
                 newlines.                                              [string]

graphile-migrate uncommit

graphile-migrate uncommit

This command is useful in development if you need to modify your latest commit
before you push/merge it, or if other DB commits have been made by other
developers and you need to 'rebase' your migration onto theirs. Moves the latest
commit out of the committed migrations folder and back to the current migration
(assuming the current migration is empty-ish). Removes the migration tracking
entry from ONLY the local database. Do not use after other databases have
executed this committed migration otherwise they will fall out of sync. Assuming
nothing else has changed, `graphile-migrate uncommit && graphile-migrate commit`
should result in the exact same hash. Development only, and liable to cause
conflicts with other developers - be careful.

Options:
  --help        Show help                                              [boolean]
  --config, -c  Optional path to gmrc file        [string] [default: .gmrc[.js]]

graphile-migrate reset

graphile-migrate reset

Drops and re-creates the database, re-running all committed migrations from the
start. **HIGHLY DESTRUCTIVE**.

Options:
  --help        Show help                                              [boolean]
  --config, -c  Optional path to gmrc file        [string] [default: .gmrc[.js]]
  --shadow      Applies migrations to shadow DB.      [boolean] [default: false]
  --erase       This is your double opt-in to make it clear this DELETES
                EVERYTHING.                           [boolean] [default: false]

graphile-migrate status

graphile-migrate status

Exits with a bitmap status code indicating statuses:

- 1 if there are committed migrations that have not been executed yet (requires
DB connection)
- 2 if the current migration is non-empty (ignoring comments)

If both of the above are true then the output status will be 3 (1+2). If neither
are true, exit status will be 0 (success). Additional messages may also be
output.

Options:
  --help          Show help                                            [boolean]
  --config, -c    Optional path to gmrc file      [string] [default: .gmrc[.js]]
  --skipDatabase  Skip checks that require a database connection.
                                                      [boolean] [default: false]

graphile-migrate compile

graphile-migrate compile [file]

Compiles a SQL file, inserting all the placeholders and returning the result to
STDOUT

Options:
  --help        Show help                                              [boolean]
  --config, -c  Optional path to gmrc file        [string] [default: .gmrc[.js]]
  --shadow      Apply shadow DB placeholders (for development).
                                                      [boolean] [default: false]

graphile-migrate run

graphile-migrate run [file]

Compiles a SQL file, inserting all the placeholders, and then runs it against
the database. Useful for seeding. If called from an action will automatically
run against the same database (via GM_DBURL envvar) unless --shadow or
--rootDatabase are supplied.

Options:
  --help          Show help                                            [boolean]
  --config, -c    Optional path to gmrc file      [string] [default: .gmrc[.js]]
  --shadow        Apply to the shadow database (for development).
                                                      [boolean] [default: false]
  --root          Run the file using the root user (but application database).
                                                      [boolean] [default: false]
  --rootDatabase  Like --root, but also runs against the root database rather
                  than application database.          [boolean] [default: false]

Configuration

Configuration can be stored in a .gmrc JSON5 file (compatible with JSON and JSONC), or in a .gmrc.js file which will be require()'d. The following configuration options are available:

• connectionString (or DATABASE_URL envvar) — this is your main development database. If you run graphile-migrate reset this will be dropped without warning, so be careful.
• shadowConnectionString (or SHADOW_DATABASE_URL envvar) — the shadow database which will be dropped frequently, so don't store anything to it that you care about.
• rootConnectionString (or ROOT_DATABASE_URL envvar) — this is used to connect to the database server with superuser (or superuser-like) privileges to drop and re-create the relevant databases (via the reset command directly, or via the commit command for the shadow database). It must not be a connection to the database in connectionString or shadowConnectionString. It defaults to "template1" if the key or environment variable is not set so it may result in PG connection errors if a default PG template1 database is not available.
• pgSettings — optional string-string key-value object defining settings to set in PostgreSQL when migrating. Useful for setting search_path for example. Beware of changing this, a full reset will use the new values which may lead to unexpected consequences.
• placeholders — optional string-string key-value object defining placeholder values to be replaced when encountered in any migration files. Placeholders must begin with a colon and a capital letter, and then can continue with a string of capital letters, numbers and underscores /^:[A-Z][A-Z0-9_]+$/. :DATABASE_NAME and :DATABASE_OWNER are automatically added to this object. The value must be a valid in the place you use it (i.e. ensure you escape the values) — graphile-migrate does not perform any escaping for you. The special value !ENV will tell graphile-migrate to load the setting from the environment variable with the same name.
• beforeReset — optional list of actions to execute before deleting and recreating the database.
• afterReset — optional list of actions to execute after the database has been created but before the migrations run, useful to set default permissions, install extensions or install external schemas like graphile-worker that your migrations may depend on. See "Actions" below.
• beforeAllMigrations — optional list of actions to execute before any pending migrations are executed.
• afterAllMigrations — optional list of actions to execute after all the migrations have ran, useful for performing a tasks like dumping the database or regenerating dependent data (GraphQL schema, type definitions, etc). See "Actions" below.
• beforeCurrent — optional list of actions to execute before current.sql is executed.
• afterCurrent — optional list of actions to execute after current.sql is loaded into the database. See "Actions" below.
• manageGraphileMigrateSchema (defaults to true) — if set to false, you assume responsibility for managing the graphile_migrate schema. Not recommended. This is useful in environments where the user running the migrations isn't granted schema creation privileges. If you set this to false, you must be sure to migrate the graphile_migrate database schema any time you update the graphile-migrate module.
• blankMigrationContent ─ what should be written to the current migration after commit. NOTE: this should only contain comments such that the current commit is "empty-ish" on creation.
• migrationsFolder ─ allows you to override where migrations are stored; defaults to ./migrations.

What follows is an example configuration file that depends on the following environmental variables being set:

• ROOT_DATABASE_URL - equivalent to rootConnectionString above, e.g. postgres://localhost/template1
• DATABASE_URL - equivalent to connectionString above, e.g. postgres://my_user:my_password@localhost/my_db
• SHADOW_DATABASE_URL - equivalent to shadowConnectionString above, e.g. postgres://my_user:my_password@localhost/my_db_shadow (should use same credentials as the )

{
  "pgSettings": {
    "search_path": "app_public,app_private,app_hidden,public"
  },
  "placeholders": {
    ":DATABASE_AUTHENTICATOR": "!ENV",
    ":DATABASE_VISITOR": "!ENV"
  },
  "afterReset": [
    "afterReset.sql",
    { "_": "command", "command": "npx --no-install graphile-worker --once" }
  ],
  "afterAllMigrations": [
    {
      "_": "command",
      "command": "pg_dump --schema-only --no-owner --exclude-schema=graphile_migrate --file=data/schema.sql \"$GM_DBURL\""
    }
  ],
  "afterCurrent": ["afterCurrent.sql"]
}

A .gmrc.js configuration file could be identical to the above, except the opening brace { would be prepended with module.exports =:

module.exports = {

All commands accept an optional --config parameter with a custom path to a .gmrc(.js) file. This is useful if, for example, you have a monorepo or other project with multiple interacting databases.

Windows

Since committed migrations utilize hashes to verify file integrity, the difference between LF and CRLF line endings on *nix and Windows will cause the hash verification to fail. Git's default/recommended approach to line endings is to convert back and forth depending on your platform. To work around this, we recommend adding a .gitattributes file to force LF line endings for the committed migrations on all platforms:

migrations/committed/*.sql text eol=lf
migrations/current.sql text eol=lf

After committing this change, you may run git checkout-index --force --all to rewrite the working copy with LF line endings. If that command does not replace the CRLF line endings, you may need to delete your copy of the repo and re-clone.

Actions

We support certain "actions" after certain events happen; for example see afterReset, afterAllMigrations and afterCurrent mentioned above. Actions should be specified as a list of strings or action spec objects.

Actions spec strings

String values are converted to sql action specs (see below) with the file property set to the string. I.e. they indicate a file within the migrations folder to execute against the database.

Action spec objects

Action spec objects are plain JSON objects with the following properties:

• _ - specifies the type of object (see supported types below)
• shadow (optional) - if set, must be a boolean; true indicates the action should only occur against the shadow DB, false indicates that the action should not occur against the shadow DB, unset runs against both databases

Each action spec subtype can have its own properties.

sql action spec

e.g.

{
  "_": "sql",
  "file": "install_extensions.sql",
  "root": false
}

The file indicates the name of a SQL file in the migrations/ folder to execute against the database (e.g. to set permissions, load data, install extensions, etc).

The root property should be used with care, and is only supported by the afterReset hook (all other hooks will throw an error when it is set). When true, the file will be run using the superuser role (i.e. the one defined in rootConnectionString) but with the database name from connectionString. This is primarily useful for creating extensions.

An identical effect can be achieved using the shorthand syntax of prepending the file name with an exclamation point, like so:

"afterReset": [ "!install_extensions.sql" ]

command action spec

e.g.

{
  "_": "command",
  "command": "npx --no-install graphile-worker --once"
}

command actions specify shell actions (e.g. running an external command such as graphile-worker which might install a separately managed worker schema into the database, or running something like pg_dump to dump the schema).

When the command is invoked it will have access to the following envvars:

• GM_DBURL - the relevant database URL (e.g. the one that was just reset/migrated)
• GM_DBNAME - the database name in GM_DBURL; you might use this if you need to use separate superuser credentials to install extensions against the database
• GM_DBUSER - the database user in GM_DBURL
• GM_SHADOW - set to 1 if we're dealing with the shadow DB, unset otherwise

IMPORTANT NOTE the DATABASE_URL envvar will be set to the nonsense value postgres://PLEASE:USE@GM_DBURL/INSTEAD to avoid ambiguity - you almost certainly mean to use GM_DBURL in your scripts since they will want to change whichever database was just reset/migrated/etc (which could be the shadow DB).

Collaboration

The intention is that developers can work on different migrations in parallel, and can switch between git branches - idempotent migrations using CASCADE when dropping should make it possible to do this with little issue (other than the implicit data loss of dropping tables/columns/etc).

graphile-migrate commit, on the other hand, should be linear - one way to approach this is to only commit a migration immediately before it is merged to master. Another approach is to do the commit on master itself. Non-linear migration commits will result in errors, and may lead to you resetting your development database.

Idempotency

graphile-migrate is all about iteration; you write your database modification commands in migrations/current.sql and every time you save it is ran against the database, generally taking under 100ms.

Because we run the same script over and over (on every save) and there's no down migrations, you need to make your script idempotent. PostgreSQL has a number of idempotent commands such as:

create or replace function...
drop table if exists ...
drop trigger if exists ...
-- etc

When these aren't suitable you can start your migration with an explicit rollback: commands that undo later actions. For example:

-- undo
drop table if exists people;

-- redo
create table people (
  id serial primary key,
  name text
);

When it comes time to commit your migration we will run it against a "shadow" database to make sure it's valid.

It's often wise to use DROP ... CASCADE so that if other migrations are worked on in parallel no additional rollback step is required. When you DROP ... CASCADE, be sure to add back any dropped dependents (triggers, indexes, etc) once the dropped entity has been replaced. Reviewing the database schema diff can help you spot these issues.

More examples of idempotent operations can be found in docs/idempotent-examples.md.

Disable Transaction

Some migrations require execution outside of a transaction (e.g. to enable augmenting non-DDL-safe things, such as ENUMs in PostgreSQL). To disable wrapping a given migration file in a transaction, use the special comment --! no-transaction at the top of the migration file, e.g.

--! no-transaction
ALTER TYPE user_role ADD VALUE IF NOT EXISTS 'Admin';

IMPORTANT: pg always runs multi-statement queries in a pseudo-transaction, so --! no-transaction migrations must contain exactly one statement. You might be able to work around this with a DO $ block? (If this works, please send a PR to this paragraph.)

Editing a committed migration

Graphile Migrate deliberately performs cryptographic hashing to avoid/detect accidental editing of committed migrations and to ensure there is a strict linear progression in migrations. By default, Graphile Migrate will refuse to run a migration if its hash does not match what it declares; this is generally desired (and you shouldn't have to worry about it).

Should you need to go back and edit a committed migration you can opt out of Graphile Migrate's consistency checks by adding the comment --! AllowInvalidHash to the very top of the committed migration. Please note that editing the migration WILL NOT cause the migration to run again on yours or any other system.

The need to edit a previous migration generally arises if there was a mistake in your migration that prevents it running on production but you don't want to reset your staging database, or where an update to PostgreSQL has made the syntax or commands in an older migration invalid and thus you must edit them to make the migration run against a clean database again. Most users should never need this functionality. If you find yourself using it more than once or twice, please get in touch and we can discuss how the tool can better serve your needs.

Terminology

The current migration

The file (or files) in which the non-committed migration that would be executed by graphile-migrate watch is defined. By default this is in the migrations/current.sql file, but it might be migrations/current/*.sql if you're using folder mode.

Committed migration(s)

The files for migrations that you've committed with graphile-migrate commit (note: this is different to committing the files using your version control system, e.g. git). By default they're located in migrations/committed/*.sql and are numbered.

Root

We use the term "root" to indicate a database role with superuser or superuser-like privileges. This should include the ability to create and delete databases, but may also include the abilities to create extensions and/or roles.

Since "superuser" has a specific meaning and is not strictly required for these activities we avoid that term, however you may find that you use a superuser as your root user - this is expected.

Status

STABLE

This project is intended to be consumed via the CLI, which is stable and is being used in production in many projects. The CLI doesn't have explicit tests (PR welcome!), but it's a thin wrapper around the programmatic API which has copious tests.

The programmatic API is deliberately undocumented; it is not a public interface at this time (though it is fully typed in TypeScript). We reserve the right to make breaking changes to the programmatic API in patch releases (though this has not happened yet and is unlikely to happen without good reason). Should you need to use the programmatic API, please get in touch to encourage us to make this a supported interface ─ we'd love to know how you're using it! src/cli.ts is the best place to start.

The project as a whole is stable, but the approach is still "experimental", in particular:

• the approach of up-only and re-runnable migrations is not for the faint of heart ─ it requires solid SQL knowledge and if insufficient attention is paid it could result in your migrations and your local database state drifting apart (see 'Drift' below).

If you don't understand what makes Graphile Migrate awesome, you may want to consider an alternative migration framework such as these awesome (and quite diverse) projects:

• db-migrate
• sqitch
• Flyway
• migra

Node.js versioning policy

We only support LTS versions of Node.js; the currently supported versions are:

• Node v12.x
• Node v14.x

Other versions of Node may work, but are not officially supported.

Once a Node.js version becomes "unsupported" (i.e. the maintenance LTS window ends), this project will no longer support it either. We may drop support for unmaintained versions of Node.js in a minor release.

Drift

NOTE: drift only affects your local development database, it cannot occur in your production database assuming you're only using graphile-migrate migrate in production.

In development, if you're insufficiently careful with modifications to current.sql (including when you choose to save the file, and when switching branches in git) you may end up with a local database state that differs from what you'd expect given the committed migrations and contents of current.sql. We strongly recommend against auto-save for this reason; and recommend that you keep a dumped schema.sql to help you spot unexpected changes.

Here's an illustrative example to explain the drift phenomenon, with function inspired by XKCD221. Imagine that you're running graphile-migrate watch locally and you write the following to current.sql:

-- Revision 1
create function rnd() returns int as $
  select 4;
$ language sql stable;

Because watch runs the contents of current.sql whenever it changes, this will create the rnd() function in your local database.

A couple seconds later you change your mind, and decide to rename the function, writing the following to current.sql:

-- Revision 2
create function get_random_number() returns int as $
  select 4;
$ language sql stable;

This creates get_random_number(), but no-one ever said to delete rnd(), so now both functions exist. According to the committed migrations and current.sql only get_random_number() should exist. The existence of the orphaned rnd() function in your local database is what we term "drift" ─ this function will never appear in your production database even after you commit this latest migration; it also won't be in your shadow database (because we reset the shadow database and reapply all the migrations frequently).

Since Graphile Migrate doesn't know how to reverse the SQL you've written, it's up to you to make the SQL safe so that it can be ran over and over, and adjust to your changes. The two to current.sql versions above should have been

-- Revision 1
drop function if exists rnd();

create function rnd() returns int as $
  select 4;
$ language sql stable;

and

-- Revision 2
drop function if exists rnd();
drop function if exists get_random_number();

create function get_random_number() returns int as $
  select 4;
$ language sql stable;

TODO:

• Store pgSettings with committed transactions to protect against user edits

• Add graphile-migrate check command: reset the shadow database to the latest dump, apply the current migration to the shadow database, and output a SQL schema diff you can use to ensure no accidental changes have been made

• Add graphile-migrate import command: used after init but before running any other commands, imports the existing database as if it were the first migration. (For now just pg_dump, and put the schema in migrations/schema.sql.)