I have no idea if this is really what you’ll end up wanting, but we do have some tooling around this kind of thing. What you can do is this:

snapshot =
  YourApi
  |> AshPostgres.MigrationGenerator.take_snapshots(YourRepo, [TheResource])
  |> Enum.at(0)

snapshot_without_indices =
  some_custom_code_to_remove_indices

operations = AshPostgres.MigrationGenerator.get_operations_from_snapshots([snapshot], [snapshot_without_indices])

{up, down} = AshPostgres.MigrationGenerator.build_up_and_down(operations)

# do something with up/down here

Thinking about this, I don’t see how you could really do what you want that way…we generate ecto migration code. You’d have to define an ecto migration module and run it. If thats on the table then you could use up to remove everything and down to add them back

Hey @zachdaniel , here is a quick update regarding this issue:

I created a small resource to make troubleshot easier:

defmodule Pacman.Markets.Entity2 do
  @moduledoc false

  use Ash.Resource,
    data_layer: AshPostgres.DataLayer

  attributes do
    uuid_primary_key :id
  end

  postgres do
    table "entities"

    repo Pacman.Repo
  end

  identities do
    identity :unique_id, [:id]
  end
end

When I run the take_snapshot code, I get:

iex(87)> snapshot = Pacman.Markets |> AshPostgres.MigrationGenerator.take_snapshots(Pacman.Repo, [Pacman.Markets.Entity2]) |> Enum.at(0)
%{
  attributes: [
    %{
      default: "fragment(\"uuid_generate_v4()\")",
      size: nil,
      type: :uuid,
      source: :id,
      references: nil,
      primary_key?: true,
      allow_nil?: false,
      generated?: false
    }
  ],
  table: "entities",
  hash: "8123997EA2DF402FDFB3E8145AF8666DCD0894EEF02C235DE06B42D47AF3457A",
  repo: Pacman.Repo,
  schema: nil,
  identities: [
    %{
      name: :unique_id,
      keys: [:id],
      index_name: "entities_unique_id_index",
      all_tenants?: false,
      base_filter: nil
    }
  ],
  base_filter: nil,
  multitenancy: %{global: nil, attribute: nil, strategy: nil},
  check_constraints: [],
  custom_indexes: [],
  custom_statements: [],
  has_create_action: false
}

If I just run the rest of the code, I will get the error shown above, to make it work I needed to do the following changes:

1. Clean the attributes field array since I don’t need it and it will break;
2. Replace the repo field value from atom to string;
3. In the identities field, I changed the name field value from atom to string;
4. In the identities field, I changed the keys field values from atom to string;

After these changes, my snapshot changed to the following one:

%{
  attributes: [],
  table: "entities",
  hash: "8123997EA2DF402FDFB3E8145AF8666DCD0894EEF02C235DE06B42D47AF3457A",
  repo: "Pacman.Repo", # CHANGED
  schema: nil,
  identities: [
    %{
      name: "unique_id", # CHANGED
      keys: ["id"], # CHANGED
      index_name: "entities_unique_id_index",
      all_tenants?: false,
      base_filter: nil
    }
  ],
  base_filter: nil,
  multitenancy: %{global: nil, attribute: nil, strategy: nil},
  check_constraints: [],
  custom_indexes: [],
  custom_statements: [],
  has_create_action: false
}

After these changes, the rest of the code works great:

iex(90)> snapshot_without_indices = %{snapshot | custom_indexes: [], identities: []}
%{
  attributes: [],
  table: "entities",
  hash: "8123997EA2DF402FDFB3E8145AF8666DCD0894EEF02C235DE06B42D47AF3457A",
  repo: "Pacman.Repo",
  schema: nil,
  identities: [],
  base_filter: nil,
  multitenancy: %{global: nil, attribute: nil, strategy: nil},
  check_constraints: [],
  custom_indexes: [],
  custom_statements: [],
  has_create_action: false
}
iex(91)> 
nil
iex(92)> operations = AshPostgres.MigrationGenerator.get_operations_from_snapshots([snapshot], [snapshot_without_indices])
[
  %AshPostgres.MigrationGenerator.Operation.RemoveUniqueIndex{
    identity: %{
      name: :unique_id,
      keys: [:id],
      index_name: "entities_unique_id_index",
      all_tenants?: false,
      base_filter: nil
    },
    schema: nil,
    table: "entities",
    multitenancy: %{global: nil, attribute: nil, strategy: nil},
    old_multitenancy: %{global: nil, attribute: nil, strategy: nil},
    no_phase: true
  }
]
iex(93)> 
nil
iex(94)> {down, up} = AshPostgres.MigrationGenerator.build_up_and_down(operations)
{"drop_if_exists unique_index(:entities, [:id], name: \"entities_unique_id_index\")\n",
 "create unique_index(:entities, [:id], name: \"entities_unique_id_index\")\n"}

So, the problem is that `AshPostgres.MigrationGenerator expects that some values are string and try to convert it to an atom, but in this case they are already an atom.

An easy solution for that is just to add the following function to AshPostgres.MigrationGenerator:

  defp maybe_to_atom(value) when is_atom(value), do: value
  defp maybe_to_atom(value), do: String.to_atom(value)

And replace all String.to_atom calls in that module to use that function.

After doing that the original snapshot starts working great.

Would you accept an PR with that change?

Thanks @zachdaniel , now the migration is perfert!

Also, just in case someone wants to do something similar to this, this is the code I’m using to actually run the up and down variables generated from the code:

  import Ecto.Migration

  args = {self(), Repo, Repo.config(), nil, :forward, :down, %{level: :info, sql: true}}

  {:ok, runner} =
    DynamicSupervisor.start_child(Ecto.MigratorSupervisor, {Ecto.Migration.Runner, args})

  Ecto.Migration.Runner.metadata(runner, [])

  # NOTE: This will run the down migration
  Code.eval_string(down, [], __ENV__)
  Ecto.Migration.Runner.flush()

  # NOTE: This will run the up migration
  Code.eval_string(up, [], __ENV__)
  Ecto.Migration.Runner.flush()

  Agent.stop(runner)