Fix find_by with associations not working with adequate record

For now, we will just skip the cache when a non-column key is used in the hash.
If the future, we can probably move some of the logic in PredicateBuilder.expand
up the chain to make caching possible for association queries.

Closes #16903
Fixes #16884

2 files changed, 8 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/core.rb b/activerecord/lib/active_record/core.rb
index 68e89b981e..069aa977bf 100644
--- a/activerecord/lib/active_record/core.rb
+++ b/activerecord/lib/active_record/core.rb
@@ -162,6 +162,9 @@ module ActiveRecord
           v.nil? || Array === v || Hash === v
         }
 
+        # We can't cache Post.find_by(author: david) ...yet
+        return super unless hash.keys.all? { |k| columns_hash.has_key?(k.to_s) }
+
         key  = hash.keys
 
         klass = self
diff --git a/activerecord/test/cases/finder_test.rb b/activerecord/test/cases/finder_test.rb
index 7228a75548..ac570ecbe0 100644
--- a/activerecord/test/cases/finder_test.rb
+++ b/activerecord/test/cases/finder_test.rb
@@ -1073,6 +1073,11 @@ class FinderTest < ActiveRecord::TestCase
     assert_equal nil, Post.find_by("1 = 0")
   end
 
+  test "find_by with associations" do
+    assert_equal authors(:david), Post.find_by(author: authors(:david)).author
+    assert_equal authors(:mary) , Post.find_by(author: authors(:mary) ).author
+  end
+
   test "find_by doesn't have implicit ordering" do
     assert_sql(/^((?!ORDER).)*$/) { Post.find_by(id: posts(:eager_other).id) }
   end