preserve prepare_changes in Ecto.Changeset.merge/2

lib/ecto/changeset.ex

@@ -1549,6 +1549,8 @@ defmodule Ecto.Changeset do
     * `required` - required fields are merged; all the fields that appear
       in the required list of both changesets are moved to the required
       list of the resulting changeset.
+    * `prepare` - prepare callbacks are concatenated in the order of the
+      changesets being merged.
 
   ## Examples
 
@@ -1574,6 +1576,7 @@ defmodule Ecto.Changeset do
     new_filters = Map.merge(cs1.filters, cs2.filters)
     new_validations = cs1.validations ++ cs2.validations
     new_constraints = cs1.constraints ++ cs2.constraints
+    new_prepare = cs2.prepare ++ cs1.prepare
 
     # They are always set, so they should never be nil
     _ = merge_identical(cs1.empty_values, cs2.empty_values, "empty values")
@@ -1586,7 +1589,8 @@ defmodule Ecto.Changeset do
           filters: new_filters,
           action: new_action,
           validations: new_validations,
-          constraints: new_constraints
+          constraints: new_constraints,
+          prepare: new_prepare
       },
       cs2
     )

test/ecto/changeset_test.exs

@@ -662,6 +662,18 @@ defmodule Ecto.ChangesetTest do
     assert length(constraints(changeset)) == 2
   end
 
+  test "merge/2: merges prepare changes callbacks" do
+    data = %Post{}
+    cs1 = change(data) |> prepare_changes(&put_change(&1, :title, "Title"))
+    cs2 = change(data) |> optimistic_lock(:upvotes)
+
+    for changeset <- [merge(cs1, cs2), merge(cs2, cs1)] do
+      assert length(changeset.prepare) == 2
+      assert changeset.filters == %{upvotes: 0}
+      assert prepared_changes(changeset) == %{title: "Title", upvotes: 1}
+    end
+  end
+
   test "merge/2: merges types" do
     cs1 = cast({%{}, %{title: :string}}, %{title: "foo"}, ~w(title)a)
     cs2 = cast({%{}, %{body: :string}}, %{body: "foo"}, ~w(body)a)