[Proposal] preload function which handles {:ok, ...} or {:error, ...} tuple

[Tiago Barroso]

I think Ecto should export something like the following function preload_in_result function. The goal is to be able to easily preload results from the Repo.insert/1, Repo.update/1, etc inside pipelines.

@doc """

  Preload the associations in a database result.

  It accepts the following types of result:

    - `{:ok, struct}` where `struct` is an ecto schema

    - `{:error, changeset}` where `changeset` is a changeset

  This function is optimized to process the results of operations such as

  `c:Repo.insert/1` and `c:Repo.update/1`.

  """

  def preload_in_result({:ok, struct}, repo, preloads) do

    {:ok, repo.preload(struct, preloads)}

  end

  def preload_in_result({:error, changeset}, repo, preloads) do

    {:error, preload_in_changeset(changeset, repo, preloads)}

  end

  @doc """

  Preload the associations in a changeset

  """

  def preload_in_changeset(changeset, repo, preloads) do

    preload_in_changeset_helper(changeset, repo, preloads)

  end

  defp preload_in_changeset_helper(%Ecto.Changeset{} = changeset, repo, preloads) do

    # Preloading the data is very easy, because the data

    # is usually a struct already

    new_data = repo.preload(changeset.data, preloads)

    # Handling changes is harder.

    # We'll have to create a fake changeset.

    schema = changeset.data.__meta__.schema

    changes = changeset.changes

    # Create a "fake" struct so that we can preload the assocs.

    changes_struct = struct(schema, changes)

    changes_preloaded = repo.preload(changes_struct, preloads)

    preloaded_assocs = Map.take(changes_preloaded, preloads)

    merged_changes = Map.merge(changes, preloaded_assocs)

    # Update the data and the changesets

    %{changeset | data: new_data, changes: merged_changes}

  end

  defp preload_in_changeset_helper(list_of_changesets, repo, preloads)

        when is_list(list_of_changesets) do

    Enum.map(list_of_changesets, fn changeset ->

      preload_in_changeset_helper(changeset, repo, preloads)

    end)

  end

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