For MySQL, foreign keys are dependent on index.

db/migrate/20180223120443_create_user_interacted_projects_table.rb

@@ -10,9 +10,6 @@ class CreateUserInteractedProjectsTable < ActiveRecord::Migration
       t.references :user, null: false
       t.references :project, null: false
     end
-
-    add_concurrent_foreign_key :user_interacted_projects, :users, column: :user_id, on_delete: :cascade
-    add_concurrent_foreign_key :user_interacted_projects, :projects, column: :project_id, on_delete: :cascade
   end
 
   def down

db/post_migrate/20180223124427_build_user_interacted_projects_table.rb

@@ -14,11 +14,17 @@ class BuildUserInteractedProjectsTable < ActiveRecord::Migration
     end.up
 
     add_concurrent_index :user_interacted_projects, [:project_id, :user_id], unique: true
+
+    add_concurrent_foreign_key :user_interacted_projects, :users, column: :user_id, on_delete: :cascade
+    add_concurrent_foreign_key :user_interacted_projects, :projects, column: :project_id, on_delete: :cascade
   end
 
   def down
     execute "TRUNCATE user_interacted_projects"
 
+    remove_foreign_key :user_interacted_projects, :users
+    remove_foreign_key :user_interacted_projects, :projects
+
     remove_concurrent_index_by_name :user_interacted_projects, 'index_user_interacted_projects_on_project_id_and_user_id'
   end