Hi everyone,

I’m building out a complex database schema as part of a Phoenix app and I’m running into an error when i make updates to the migration. For example I have a :users table and :comments:

    create table(:comments) do
      add :title, :string

      timestamps()
    end

    create table(:users) do
      add :name, :string


      timestamps()
    end

I run ecto.migrate with no issue, but now I’d like to add a reference on the :comments table:

    create table(:comments) do
      add :title, :string
      add :users_id, references(:users, on_delete: :nothing)

      timestamps()
    end

Rather than rolling back the previous migration I prefer to ecto.reset, however I encounter this error.

The database for MigrationExample.Repo has been dropped
The database for MigrationExample.Repo has been created

20:42:28.605 [info]  == Running 20200126013640 MigrationExample.Repo.Migrations.CreateComments.change/0 forward

20:42:28.607 [info]  create table comments
** (Postgrex.Error) ERROR 42P01 (undefined_table) relation "users" does not exist
    (ecto_sql) lib/ecto/adapters/sql.ex:629: Ecto.Adapters.SQL.raise_sql_call_error/1
    (elixir) lib/enum.ex:1336: Enum."-map/2-lists^map/1-0-"/2
    (ecto_sql) lib/ecto/adapters/sql.ex:716: Ecto.Adapters.SQL.execute_ddl/4
    (ecto_sql) lib/ecto/migration/runner.ex:343: Ecto.Migration.Runner.log_and_execute_ddl/3
    (ecto_sql) lib/ecto/migration/runner.ex:117: anonymous fn/6 in Ecto.Migration.Runner.flush/0
    (elixir) lib/enum.ex:1948: Enum."-reduce/3-lists^foldl/2-0-"/3
    (ecto_sql) lib/ecto/migration/runner.ex:116: Ecto.Migration.Runner.flush/0
    (stdlib) timer.erl:166: :timer.tc/1
    (ecto_sql) lib/ecto/migration/runner.ex:25: Ecto.Migration.Runner.run/7
    (ecto_sql) lib/ecto/migrator.ex:342: Ecto.Migrator.attempt/7
    (ecto_sql) lib/ecto/migrator.ex:243: anonymous fn/4 in Ecto.Migrator.do_up/4
    (ecto_sql) lib/ecto/migrator.ex:324: anonymous fn/3 in Ecto.Migrator.run_maybe_in_transaction/6
    (ecto_sql) lib/ecto/adapters/sql.ex:903: anonymous fn/3 in Ecto.Adapters.SQL.checkout_or_transaction/4
    (db_connection) lib/db_connection.ex:1427: DBConnection.run_transaction/4
    (ecto_sql) lib/ecto/migrator.ex:323: Ecto.Migrator.run_maybe_in_transaction/6
    (elixir) lib/task/supervised.ex:90: Task.Supervised.invoke_mfa/2
    (elixir) lib/task/supervised.ex:35: Task.Supervised.reply/5
    (stdlib) proc_lib.erl:249: :proc_lib.init_p_do_apply/3

The issue is that the order of the migrations is important and the :comments table is being created before the :users table, and therefore throwing the error. Is there a way to resolve this without manually specifying the repo in the migration ecto.migrate -r or rolling back a migration. My preference is to always reset the database.

Thanks for the help