Fix select_merge on map with outer join (#3219)

Resolves #3218

Three cases have been added to merge processing:

- If the merge argument (`right`) is `nil`, return `left` unmodified. This can
  happen with a `:left_join` or `:full_join`.

- If the merge source (`left`) is `nil`, return `right` unmodified. This can
  happen with a `:right_join` or `:full_join`.

- If both the source and argument are `nil`, return `nil`. This _probably_
  shouldn’t happen, but is currently included for completeness.

integration_test/cases/repo.exs

@@ -1237,7 +1237,8 @@ defmodule Ecto.Integration.RepoTest do
     end
 
     test "merge" do
-      %Post{} = TestRepo.insert!(%Post{title: "1", counter: nil})
+      date = Date.utc_today()
+      %Post{id: post_id} = TestRepo.insert!(%Post{title: "1", counter: nil, posted: date, public: false})
 
       # Merge on source
       assert [%Post{title: "2"}] =
@@ -1256,6 +1257,35 @@ defmodule Ecto.Integration.RepoTest do
              Post |> select([p], merge(%{title: p.title}, %{title: "2"})) |> TestRepo.all()
       assert [%{title: "2"}] =
              Post |> select([p], %{title: p.title}) |> select_merge([p], %{title: "2"}) |> TestRepo.all()
+
+      # Merge on outer join with map
+      %Permalink{} = TestRepo.insert!(%Permalink{post_id: post_id, url: "Q", title: "Z"})
+
+      # left join record is present
+      assert [%{url: "Q", title: "1", posted: date}] =
+               Permalink
+               |> join(:left, [l], p in Post, on: l.post_id == p.id)
+               |> select([l, p], merge(l, map(p, ^~w(title posted)a)))
+               |> TestRepo.all()
+
+      assert [%{url: "Q", title: "1", posted: date}] =
+               Permalink
+               |> join(:left, [l], p in Post, on: l.post_id == p.id)
+               |> select_merge([_l, p], map(p, ^~w(title posted)a))
+               |> TestRepo.all()
+
+      # left join record is not present
+      assert [%{url: "Q", title: "Z", posted: nil}] =
+               Permalink
+               |> join(:left, [l], p in Post, on: l.post_id == p.id and p.public)
+               |> select([l, p], merge(l, map(p, ^~w(title posted)a)))
+               |> TestRepo.all()
+
+      assert [%{url: "Q", title: "Z", posted: nil}] =
+               Permalink
+               |> join(:left, [l], p in Post, on: l.post_id == p.id and p.public)
+               |> select_merge([_l, p], map(p, ^~w(title posted)a))
+               |> TestRepo.all()
     end
 
     test "merge with update on self" do

integration_test/support/schemas.exs

@@ -1,12 +1,14 @@
-Code.require_file "types.exs", __DIR__
+Code.require_file("types.exs", __DIR__)
 
 defmodule Ecto.Integration.Schema do
   defmacro __using__(_) do
     quote do
       use Ecto.Schema
+
       type =
         Application.get_env(:ecto, :primary_key_type) ||
-        raise ":primary_key_type not set in :ecto application"
+          raise ":primary_key_type not set in :ecto application"
+
       @primary_key {:id, type, autogenerate: true}
       @foreign_key_type type
     end
@@ -104,14 +106,16 @@ defmodule Ecto.Integration.Permalink do
 
   schema "permalinks" do
     field :url, :string, source: :uniform_resource_locator
+    field :title, :string
+    field :posted, :date, virtual: true
     belongs_to :post, Ecto.Integration.Post, on_replace: :nilify
     belongs_to :update_post, Ecto.Integration.Post, on_replace: :update, foreign_key: :post_id, define_field: false
     belongs_to :user, Ecto.Integration.User
     has_many :post_comments_authors, through: [:post, :comments_authors]
   end
 
   def changeset(schema, params) do
-    Ecto.Changeset.cast(schema, params, [:url])
+    Ecto.Changeset.cast(schema, params, [:url, :title])
   end
 end
 

lib/ecto/query.ex

@@ -1232,6 +1232,21 @@ defmodule Ecto.Query do
   being merged on must be a struct or a map. If it is a struct, the fields
   merged later on must be part of the struct, otherwise an error is raised.
 
+  If the argument to `:select_merge` is a constructed struct (`struct/2`) or
+  map (`map/2`) where the source to `struct/2` or `map/2` may be a `nil` value
+  (as in an outer join), the source will be returned unmodified.
+
+      query =
+        Post
+        |> join(:left, [p], t in Post.Translation,
+          on: t.post_id == p.id and t.locale == ^"en"
+        )
+        |> select_merge([_p, t], map(t, ^~w(title summary)a))
+
+  If there is no English translation for the post, the untranslated post
+  `title` will be returned and `summary` will be `nil`. If there is, both
+  `title` and `summary` will be the value from `Post.Translation`.
+
   `select_merge` cannot be used to set fields in associations, as
   associations are always loaded later, overriding any previous value.
   """

lib/ecto/repo/queryable.ex

@@ -279,6 +279,15 @@ defmodule Ecto.Repo.Queryable do
         {%{}, %{}} ->
           Map.merge(left, right)
 
+        {%{}, nil} ->
+          left
+
+        {nil, %{}} ->
+          right
+
+        {nil, nil} ->
+          nil
+
         {_, %{}} ->
           raise ArgumentError,
                 "cannot merge because the left side is not a map, got: #{inspect(left)}"