Handling Has-Many Through Cascading Deletes in Rails

On the stream, we’re building an event platform using Ruby on Rails. In this week’s past stream (see below), we ran into a problem with cascading deletes while running our tests. This issue stemmed from a misconfiguration of a “has_many through” relationship in our models.

To focus on this problem, we’ll only look at the relevant parts of our application. This includes the Event, EventSession, EventSpeaker, and EventSessionSpeaker models. In our application, an Event has many EventSessions and EventSpeakers. An EventSession has many EventSessionSpeakers, and an EventSpeaker has many EventSessionSpeakers. This means we have 2 1-to-many relations and 1 many-to-many relation. Our reduced schema to focus on the relationships looks like this:

ActiveRecord::Schema[7.1].define(version: 2024_05_31_000328) do
  create_table "event_session_speakers", id: false, force: :cascade do |t|
    t.string "event_session_id", null: false
    t.string "event_speaker_id", null: false
    t.index ["event_session_id"], name: "index_event_session_speakers_on_event_session_id"
    t.index ["event_speaker_id"], name: "index_event_session_speakers_on_event_speaker_id"
  end

  create_table "event_sessions", id: :string, default: -> { "gen_random_uuid()" }, force: :cascade do |t|
    # other fields here
    t.string "event_id"
    t.index ["event_id"], name: "index_event_sessions_on_event_id"
  end

  create_table "event_speakers", id: :string, default: -> { "gen_random_uuid()" }, force: :cascade do |t|
    # other fields here
    t.string "event_id"
    t.index ["event_id"], name: "index_event_speakers_on_event_id"
  end

  create_table "events", id: :string, default: -> { "gen_random_uuid()" }, force: :cascade do |t|
    # other fields here
  end

Our model definitions were as follows:

class Event < ApplicationRecord
  has_many :event_sessions, dependent: :destroy
  has_many :event_speakers, dependent: :destroy
end

class EventSession < ApplicationRecord
  belongs_to :event
  has_many :event_session_speakers, dependent: :destroy
  has_many :event_speakers, through: :event_session_speakers
end


class EventSpeaker < ApplicationRecord
  belongs_to :event
  has_many :event_session_speakers, dependent: :destroy
  has_many :event_sessions, through: :event_session_speakers
end


class EventSessionSpeaker < ApplicationRecord
  belongs_to :event_session
  belongs_to :event_speaker
end

When we were running an integration test to delete an Event, we were receiving an error that a column event_session_speaker was not found. The problem existed because of how we defined our dependent option on EventSession and EventSpeaker. For those unfamiliar, the dependent option allows you to specify what happens to associated records when the parent record is deleted. The :destroy option we used will delete the associated records when the parent record is deleted and run any associated callbacks. In this case, the EventSession and EventSpeaker models were both attempting to delete their associated EventSessionSpeakers records. This tried to run the query:

TRANSACTION (0.1ms)  begin transaction
  EventSession Load (0.1ms)  SELECT "event_sessions".* FROM "event_sessions" WHERE "event_sessions"."event_id" = ?  [["event_id", "30663738"]]
  EventSessionSpeaker Load (0.1ms)  SELECT "event_session_speakers".* FROM "event_session_speakers" WHERE "event_session_speakers"."event_session_id" = ?  [["event_session_id", "42880873"]]
  EventSessionSpeaker Destroy (0.2ms)  DELETE FROM "event_session_speakers" WHERE "event_session_speakers"."" IS NULL
TRANSACTION (0.0ms)  rollback transaction

As you can see, the delete query is trying to delete all EventSessionSpeakers where the event_session_speakers column is NULL. This is because the EventSession and EventSpeaker models are trying to delete their associated EventSessionSpeakers records. However, the EventSessionSpeaker model is trying to delete the record twice because it is associated with both the EventSession and EventSpeaker models. This is why the event_session_speakers column is NULL in the query and making the query invalid.

To fix this, we need to update the dependent option on the EventSession and EventSpeaker models. We need to change the dependent option to :delete_all. This will delete the associated records when the parent record is deleted without running any associated callback. This will prevent the duplicated deletions by letting EventSession and EventSpeaker handle their cleanup individually. So, we updated our models to be:

class EventSession < ApplicationRecord
  belongs_to :event
  has_many :event_session_speakers, dependent: :delete_all
  has_many :event_speakers, through: :event_session_speakers
end


class EventSpeaker < ApplicationRecord
  belongs_to :event
  has_many :event_session_speakers, dependent: :delete_all
  has_many :event_sessions, through: :event_session_speakers
end

Now, when we run the test, we see the following queries ran:

TRANSACTION (0.1ms)  begin transaction
  EventSession Load (0.2ms)  SELECT "event_sessions".* FROM "event_sessions" WHERE "event_sessions"."event_id" = ?  [["event_id", "30663738"]]
  EventSessionSpeaker Delete All (0.1ms)  DELETE FROM "event_session_speakers" WHERE "event_session_speakers"."event_session_id" = ?  [["event_session_id", "42880873"]]
  EventSession Destroy (0.1ms)  DELETE FROM "event_sessions" WHERE "event_sessions"."id" = ?  [["id", "42880873"]]
  EventSessionSpeaker Delete All (0.0ms)  DELETE FROM "event_session_speakers" WHERE "event_session_speakers"."event_session_id" = ?  [["event_session_id", "472205080"]]
  EventSession Destroy (0.0ms)  DELETE FROM "event_sessions" WHERE "event_sessions"."id" = ?  [["id", "472205080"]]
  EventSessionSpeaker Delete All (0.0ms)  DELETE FROM "event_session_speakers" WHERE "event_session_speakers"."event_session_id" = ?  [["event_session_id", "894887044"]]
  EventSession Destroy (0.0ms)  DELETE FROM "event_sessions" WHERE "event_sessions"."id" = ?  [["id", "894887044"]]
  EventSessionSpeaker Delete All (0.0ms)  DELETE FROM "event_session_speakers" WHERE "event_session_speakers"."event_session_id" = ?  [["event_session_id", "1020543588"]]
  EventSession Destroy (0.0ms)  DELETE FROM "event_sessions" WHERE "event_sessions"."id" = ?  [["id", "1020543588"]]
  EventSpeaker Load (0.1ms)  SELECT "event_speakers".* FROM "event_speakers" WHERE "event_speakers"."event_id" = ?  [["event_id", "30663738"]]
  EventSessionSpeaker Delete All (0.1ms)  DELETE FROM "event_session_speakers" WHERE "event_session_speakers"."event_speaker_id" = ?  [["event_speaker_id", "668127263"]]
  EventSpeaker Destroy (0.0ms)  DELETE FROM "event_speakers" WHERE "event_speakers"."id" = ?  [["id", "668127263"]]
  EventSessionSpeaker Delete All (0.0ms)  DELETE FROM "event_session_speakers" WHERE "event_session_speakers"."event_speaker_id" = ?  [["event_speaker_id", "350169046"]]
  EventSpeaker Destroy (0.0ms)  DELETE FROM "event_speakers" WHERE "event_speakers"."id" = ?  [["id", "350169046"]]
  EventSessionSpeaker Delete All (0.0ms)  DELETE FROM "event_session_speakers" WHERE "event_session_speakers"."event_speaker_id" = ?  [["event_speaker_id", "1066195201"]]
  EventSpeaker Destroy (0.0ms)  DELETE FROM "event_speakers" WHERE "event_speakers"."id" = ?  [["id", "1066195201"]]
  Event Destroy (0.0ms)  DELETE FROM "events" WHERE "events"."id" = ?  [["id", "30663738"]]
  TRANSACTION (0.2ms)  commit transaction

This is taking all the associated session and speaker records and cleaning them up individually. This is the correct behavior we want and will now let us perform the cascading delete as we want.

An important note to make is that this cascading delete is suboptimal and not a function you want run in a production application. In Rails 6, they introduced the :destroy_async option which will allow you to run the dependent destroy in a background job. This will prevent the cascading delete from blocking the main thread and causing performance issues. This is an optimization we should make and demonstrate on the next stream.

This was a tricky problem to debug, and there wasn’t much documentation on the subject. I hope this post helps others who run into similar issues. If you have any questions or comments, please feel free to reach out to me on Twitter, Twitch, or comment in the above video. You can find the full source available on GitHub.