Benjamin Milde

Unnest for runtime sorted results

The recent ecto 3.5 update introduced the parameterized type Ecto.Enum, which is a great way to use atoms in elixir for signifying things like statuses or types of data. In the database those values are stored as plain strings, so they’re easy to manage there as well – until it comes to sorting by those columns.

The simplest solution for sorting not alphabetically, but by e.g. the order the statuses are applied, is likely to use an proper enum column in the database or joining a table in the db, which holds an index for each possible value in a secondary column.

Arbitrary order

But what about arbitrary sorting, which cannot be placed in the db?

For postgres there’s the possibility to build that join table dynamically with unnest instead of an actual table. The usual answer one finds on the internet most often involves using VALUES lists, but they don’t really work well with ecto queries, as they’re not easily parameterized. unnest luckily can be:

defmodule MyApp.QueryHelpers
  @doc """
  Unnest into an table format.

  ## Example

      import MyApp.QueryHelpers

      status = [:waiting, :running, :done]
      order = [1, 2, 3]

      from jobs in "jobs", 
        join: ordering in unnest(^status, ^order), 
        on: jobs.status == ordering.a,
        order_by: ordering.b

  defmacro unnest(list_a, list_b) do
    quote do
      fragment("SELECT * FROM unnest(?, ?) AS t(a, b)", unquote(list_a), unquote(list_b))

As you can see unnest does work with plain lists, which can be passed as parameters and therefore work with fragment. This allows to build up an table of arbitrary size (more columns could be done by adding additional arities for the unnest helper), which can be joined to the data in the database.

Updating pagination

Given we’re manually supplying the list of “positions” this can not only be used for read operations, but e.g. also for updates for things like position columns for data ordered in the db.

@spec update_order(%{produce_id :: integer => new_position :: integer}) :: result :: term
def update_order(new_order) do
  {ids, positions} = Enum.unzip(new_order)

  query =
      from p in Product,
          positions in unnest(
            type(^ids, {:array, :integer}),
            type(^positions, {:array, :integer})
        on: == positions.a,
        update: [set: [position: positions.b]]

    Repo.transaction(fn ->
      # Create deferable constraint like that (not `unique_index/3`):
      # execute("""
      # ALTER TABLE products
      # ADD constraint products_position_unique unique (position) deferrable;
      # """, "")
      # This allows for all rows to update before checking if everything is
      # still unique.
      Repo.query!("SET CONSTRAINTS products_position_unique DEFERRED")
      Repo.update_all(query, [])