Referencing a table via the ON condition in a join should force that table to be eager-loaded via a JOIN rather than via subsequent queries.

activerecord/lib/active_record/relation.rb

@@ -407,8 +407,19 @@ def method_missing(method, *args, &block)
     private
 
     def references_eager_loaded_tables?
+      joined_tables = arel.join_sources.map do |join|
+        if join.is_a?(Arel::Nodes::StringJoin)
+          tables_in_string(join.left)
+        else
+          [join.left.table_name, join.left.table_alias]
+        end
+      end
+
+      joined_tables += [table.name, table.table_alias]
+
       # always convert table names to downcase as in Oracle quoted table names are in uppercase
-      joined_tables = (tables_in_string(arel.join_sql) + [table.name, table.table_alias]).compact.map{ |t| t.downcase }.uniq
+      joined_tables = joined_tables.flatten.compact.map { |t| t.downcase }.uniq
+
       (tables_in_string(to_sql) - joined_tables).any?
     end
 

activerecord/test/cases/relations_test.rb

@@ -850,4 +850,19 @@ def test_removing_limit_with_options
   def test_primary_key
     assert_equal "id", Post.scoped.primary_key
   end
+
+  def test_eager_loading_with_conditions_on_joins
+    scope = Post.includes(:comments)
+
+    # This references the comments table, and so it should cause the comments to be eager
+    # loaded via a JOIN, rather than by subsequent queries.
+    scope = scope.joins(
+      Post.arel_table.create_join(
+        Post.arel_table,
+        Post.arel_table.create_on(Comment.arel_table[:id].eq(3))
+      )
+    )
+
+    assert scope.eager_loading?
+  end
 end