update.ex

User
|> prepared(user_id: user_id, username: username)
|> insert(user_id: :user_id, username: :username)
|> if_not_exists
|> User.save

User
|> update(username: username)
|> where(user_id: user_id)
|> constrain(username: previous_username)
|> User.save

User
|> prepared(user_id: user_id)
|> delete(:all)  # here :all refers to all fields
|> where(user_id: :user_id)
|> User.del