Add function query/2

[joeljuca]

This issue is more of an idea than a well-formatted issue. I wrote a meta-function in one of my projects which would really work as a core SwissSchema fn. Here's now it looks overall:

defmacro __using__(_opts) do
    quote do
      import Ecto.Query, only: [from: 1, select: 3]
      alias MyApp.Repo

      @spec query(f :: (Ecto.Query.t() -> Ecto.Query.t())) ::
              {:ok, list(struct())} | {:error, term()}
      def query(f) do
        query =
          from(s in __MODULE__)
          |> select([s], s)
          |> then(f)

        {:ok, Repo.all(query)}
      rescue
        error -> {:error, error}
      end
    end
end

It's supposed to be used like this: you would pass a function of typespec (Ecto.Query.t() -> Ecto.Query.t()), receive a pre-formatted query (Ecto.Query), and augment it with your own querying criteria:

User.query(fn query ->
  query
  
  # Apply your own WHERE criteria
  |> where([u], u.username == "joeljuca")
  
  # Join the user table with other tables
  |> join(:left, [u], c in Comment, on: c.user_id == u.id)
  
  # Order, limit, etc.
  |> order_by([u], [desc: u.inserted_at])
  |> limit(100)
end)

I think this function needs some mechanism to provide a default function to prepare the query, as well as a way to call query/2 w/o this preparation (smt like User.query(fn q -> ... end, raw: true)? I'm not sure yet).

The main thing here is: I want to avoid creating very specific querying functions (eg.: get_user_by_this/1, get_user_by_that/1, get_users_by_smt_else/2`, etc.), and I'd like to avoid creating these complex functions with a huge map with params for each situation:

def get_users(%{} = params) do
  query = 
    params
    |> handle_param_A_if_present()
    |> handle_param_B_if_present()
    |> handle_param_C_if_present()
    |> handle_param_D_if_present()
    |> handle_param_E_if_present()
    |> handle_param_F_if_present()
    |> handle_param_G_if_present()
    # (...)
    |> prepare_query()


  with {...} <- query |> Repo.all() do
    # (...)
  end
end

I never loved this function style! The reason? Well, it's pretty much a DSL (Domain-Specific Language) for a single function! You need to remember the syntax of all these params and their specifics – and I never recall which one goes where, and their specific syntax, etc.

Well, Ecto is awesome – and Ecto.Query is an amazing DSL for writing SQL code in Elixir. I'd like to just using wherever and whenever I need. Hence query/2 was born. 😄

[joeljuca]

Note to myself: remember to support other SwissSchema features (eg.: custom repos).