enhance distinct error

Author: abdelhalim97

lib/ecto/adapters/myxql/connection.ex

@@ -99,7 +99,7 @@ if Code.ensure_loaded?(MyXQL) do
     ## Query
 
     @parent_as __MODULE__
-    alias Ecto.Query.{BooleanExpr, ByExpr, JoinExpr, QueryExpr, WithExpr}
+    alias Ecto.Query.{BooleanExpr, ByExpr, JoinExpr, QueryExpr, SelectExpr, WithExpr}
 
     @impl true
     def all(query, as_prefix \\ []) do
@@ -346,6 +346,15 @@ if Code.ensure_loaded?(MyXQL) do
     defp distinct(%ByExpr{expr: true}, _sources, _query), do: "DISTINCT "
     defp distinct(%ByExpr{expr: false}, _sources, _query), do: []
 
+    defp distinct(
+           %ByExpr{expr: exprs},
+           _sources,
+           query = %Ecto.Query{select: %SelectExpr{expr: {:&, [], [0]}}}
+         )
+         when is_list(exprs) do
+      error!(query, "To apply DISTINCT to multiple columns at once, use distinct: true")
+    end
+
     defp distinct(%ByExpr{expr: exprs}, _sources, query) when is_list(exprs) do
       error!(query, "DISTINCT with multiple columns is not supported by MySQL")
     end

test/ecto/adapters/myxql_test.exs

@@ -384,6 +384,15 @@ defmodule Ecto.Adapters.MyXQLTest do
     query = Schema |> distinct(false) |> select([r], {r.x, r.y}) |> plan()
     assert all(query) == ~s{SELECT s0.`x`, s0.`y` FROM `schema` AS s0}
 
+    assert_raise Ecto.QueryError,
+                 ~r"To apply DISTINCT to multiple columns at once, use distinct: true",
+                 fn ->
+                   query =
+                     Schema |> distinct([r], [r.x, r.y]) |> select([r], r) |> plan()
+
+                   all(query)
+                 end
+
     assert_raise Ecto.QueryError,
                  ~r"DISTINCT with multiple columns is not supported by MySQL",
                  fn ->