cope with distinct: true

Author: dvv

lib/ecto/adapters/postgres/connection.ex

@@ -533,6 +533,16 @@ if Code.ensure_loaded?(Postgrex) do
       do: "TRUE"
 
     defp select_fields(fields, sources, query) do
+      # NB: append order_by fields if distinct: true specified
+      fields = case query.distinct do
+        %Ecto.Query.ByExpr{expr: true} ->
+          order_by_fields = Enum.map(query.order_bys, & Keyword.values(&1.expr)) |> List.flatten()
+          Enum.reduce(order_by_fields, fields, fn
+            {{:., _, [{:&, [], [idx]}, _]}, _, _} = field, acc when idx > 0 -> acc ++ [field]
+            _field, acc -> acc
+          end)
+        _ -> fields
+      end
       Enum.map_intersperse(fields, ", ", fn
         {:&, _, [idx]} ->
           case elem(sources, idx) do

test/ecto/adapters/postgres_test.exs

@@ -579,6 +579,20 @@ defmodule Ecto.Adapters.PostgresTest do
              ~s{SELECT DISTINCT ON (s0."x") s0."x" FROM "schema" AS s0 ORDER BY s0."x" DESC}
   end
 
+  test "distinct true with assocs" do
+    query =
+      Schema |> join(:inner, [r], assoc(r, :permalink)) |> order_by([r, p], [p.id]) |> distinct([r], true) |> select([r], r.x) |> plan()
+
+    assert all(query) ==
+             ~s{SELECT DISTINCT s0."x", s1."id" FROM "schema" AS s0 INNER JOIN "schema3" AS s1 ON s1."id" = s0."y" ORDER BY s1."id"}
+
+    query =
+      Schema |> join(:inner, [r], assoc(r, :permalink)) |> order_by([r], [r.id]) |> distinct([r], true) |> select([r], r.x) |> plan()
+
+    assert all(query) ==
+             ~s{SELECT DISTINCT s0."x" FROM "schema" AS s0 INNER JOIN "schema3" AS s1 ON s1."id" = s0."y" ORDER BY s0."id"}
+  end
+
   test "coalesce" do
     query = Schema |> select([s], coalesce(s.x, 5)) |> plan()
     assert all(query) == ~s{SELECT coalesce(s0."x", 5) FROM "schema" AS s0}