Znack / beam-migration-issue

Demonstration of the problem with foreign key to outdated table

beam-migration-issue

Demonstration of the problem with foreign key to outdated table Problem discussion: haskell-beam/beam#253

Short description

Beam now has a problem with migrations if one changes the table (add column for example) and there was some dependant tables. Example provided where we have User and Post tables, Post has a link to User, then we add column to user. The problem now with link from Post to User, because it leads to outdated version of User. As a solution suggested by @tathougies I have parameterized Post (see src/Schema/Migrations/V0001ExampleBlog.hs). Now there is no way to compose migrations together.

Reproduce

Start PostgreSQL server, connect with psql postgres and then in psql shell:

CREATE DATABASE beam_demoblog;
CREATE USER demoblog;
GRANT ALL PRIVILEGES ON DATABASE beam_demoblog TO demoblog;

Then start GHCi repl with stack repl and compilation should fail:

$ stack repl

[1 of 5] Compiling Schema.Migrations.V0001ExampleBlog ( src/Schema/Migrations/V0001ExampleBlog.hs, interpreted )

src/Schema/Migrations/V0001ExampleBlog.hs:64:3: error:
    • Couldn't match type ‘user’ with ‘UserT’
      ‘user’ is a rigid type variable bound by
        the type signature for:
          migration :: forall (user :: (* -> *) -> *).
                       (Typeable user, Beamable (PrimaryKey user)) =>
                       ()
                       -> Migration
                            PgCommandSyntax
                            (CheckedDatabaseSettings Postgres (DemoblogDb user))
        at src/Schema/Migrations/V0001ExampleBlog.hs:(60,1)-(62,83)
      Expected type: Migration
                       PgCommandSyntax
                       (CheckedDatabaseSettings Postgres (DemoblogDb user))
        Actual type: free-5.0.2:Control.Monad.Free.Church.F
                       (MigrationF PgCommandSyntax)
                       (DemoblogDb
                          UserT (CheckedDatabaseEntity Postgres (DemoblogDb user)))
    • In the expression:
        DemoblogDb
          <$>
            createTable
              "user"
              (User
                 (field "user_id" serial)
                 (field "name" (varchar (Just 255)) notNull))
          <*>
            createTable
              "post"
              (Post
                 (field "post_id" serial)
                 (field "content" text notNull)
                 (UserId (field "user_id" smallint)))
      In an equation for ‘migration’:
          migration ()
            = DemoblogDb
                <$>
                  createTable
                    "user"
                    (User
                       (field "user_id" serial)
                       (field "name" (varchar (Just 255)) notNull))
                <*>
                  createTable
                    "post"
                    (Post
                       (field "post_id" serial)
                       (field "content" text notNull)
                       (UserId (field "user_id" smallint)))
    • Relevant bindings include
        migration :: ()
                     -> Migration
                          PgCommandSyntax
                          (CheckedDatabaseSettings Postgres (DemoblogDb user))
          (bound at src/Schema/Migrations/V0001ExampleBlog.hs:63:1)
   |
64 |   DemoblogDb <$>
   |   ^^^^^^^^^^^^^^...

This error is quite predictable: we have to use concrete Primary Key of the UserT entity for createTable, but the whole first migration function should be polymorphic. So we had to have some way to declare foreign key to the polymorphic entity. See the migration function in src/Schema/Migrations/V0001ExampleBlog.hs