implement support for count(_, :distinct)

Author: aseigo

lib/ecto/adapters/sqlite3/connection.ex

@@ -1437,6 +1437,10 @@ defmodule Ecto.Adapters.SQLite3.Connection do
 
   defp expr({:count, _, []}, _sources, _query), do: "count(*)"
 
+  defp expr({:count, _, [expr, :distinct]}, sources, query) do
+    ["count(distinct ", expr(expr, sources, query), ")"]
+  end
+
   defp expr({:count, _, [{:&, _, [_]}]}, _sources, query) do
     raise Ecto.QueryError,
       query: query,

test/ecto/adapters/sqlite3/connection/aggregates_test.exs

@@ -28,13 +28,13 @@ defmodule Ecto.Adapters.SQLite3.Connection.AggregatesTest do
     end
   end
 
-  test "raises when trying to distinct count" do
-    assert_raise Ecto.QueryError, fn ->
+  test "distinct counts" do
+    query =
       Schema
       |> select([r], count(r.x, :distinct))
       |> plan()
-      |> all()
-    end
+
+    assert ~s{SELECT count(distinct s0."x") FROM "schema" AS s0} == all(query)
   end
 
   test "allows naked count" do