Update Ecto.Changeset.get_assoc/3 typespecs (#4495)

Author: cserino

lib/ecto/changeset.ex

@@ -1689,7 +1689,8 @@ defmodule Ecto.Changeset do
       [%Post{id: 1, title: "world"}]
 
   """
-  @spec get_assoc(t, atom, :changeset | :struct) :: [t | Ecto.Schema.t()]
+  @spec get_assoc(t, atom, :changeset | :struct) ::
+          [t | Ecto.Schema.t()] | t | Ecto.Schema.t() | nil
   def get_assoc(changeset, name, as \\ :changeset)
 
   def get_assoc(%Changeset{} = changeset, name, :struct) do

test/ecto/changeset_test.exs

@@ -930,6 +930,23 @@ defmodule Ecto.ChangesetTest do
            ]
 
     assert get_assoc(base_changesset, :comments, :struct) == [%Comment{id: 123}]
+
+    # for a "belongs to" assoc, get_assoc returns the changeset directly
+    comment_with_post_changeset = change(%Comment{post: %Post{}})
+    assert get_assoc(comment_with_post_changeset, :post, :changeset) == change(%Post{})
+    assert get_assoc(comment_with_post_changeset, :post, :struct) == %Post{}
+
+    # empty/nil assoc behavior:
+
+    # for a "has many" assoc, get_assoc returns an empty list
+    has_many_changeset = change(%Post{})
+    assert get_assoc(has_many_changeset, :comments, :changeset) == []
+    assert get_assoc(has_many_changeset, :comments, :struct) == []
+
+    # for a "belongs to" assoc, get_assoc returns nil
+    belongs_to_changeset = change(%Comment{})
+    assert get_assoc(belongs_to_changeset, :post, :changeset) == nil
+    assert get_assoc(belongs_to_changeset, :post, :struct) == nil
   end
 
   test "fetch_change/2" do