Avoid duplicate order_by with distinct, closes #318

Author: josevalim

lib/ecto/adapters/postgres/connection.ex

@@ -505,10 +505,13 @@ if Code.ensure_loaded?(Postgrex) do
     defp order_by(%{order_bys: []}, _distinct, _sources), do: []
     defp order_by(%{order_bys: order_bys} = query, distinct, sources) do
       order_bys = Enum.flat_map(order_bys, & &1.expr)
-      [" ORDER BY " |
-       intersperse_map(distinct ++ order_bys, ", ", &order_by_expr(&1, sources, query))]
+      order_bys = order_by_concat(distinct, order_bys)
+      [" ORDER BY " | intersperse_map(order_bys, ", ", &order_by_expr(&1, sources, query))]
     end
 
+    defp order_by_concat([head | left], [head | right]), do: [head | order_by_concat(left, right)]
+    defp order_by_concat(left, right), do: left ++ right
+
     defp order_by_expr({dir, expr}, sources, query) do
       str = expr(expr, sources, query)
 

test/ecto/adapters/postgres_test.exs

@@ -309,6 +309,9 @@ defmodule Ecto.Adapters.PostgresTest do
     query = Schema |> order_by([r], [r.y]) |> distinct([r], desc: r.x) |> select([r], r.x) |> plan()
     assert all(query) == ~s{SELECT DISTINCT ON (s0."x") s0."x" FROM "schema" AS s0 ORDER BY s0."x" DESC, s0."y"}
 
+    query = Schema |> order_by([r], desc: r.x) |> distinct([r], desc: r.x) |> select([r], r.x) |> plan()
+    assert all(query) == ~s{SELECT DISTINCT ON (s0."x") s0."x" FROM "schema" AS s0 ORDER BY s0."x" DESC}
+
     query = Schema |> order_by([r], [r.y]) |> distinct([r], desc_nulls_last: r.x) |> select([r], r.x) |> plan()
     assert all(query) == ~s{SELECT DISTINCT ON (s0."x") s0."x" FROM "schema" AS s0 ORDER BY s0."x" DESC NULLS LAST, s0."y"}
   end