Solution for tracking has_and_belongs_to_many relations

[mikwat]

Currently, has_and_belongs_to_many relations are not tracked consistently. For example:

Given the following models:

class Post
  include Mongoid::Document
  include Mongoid::Timestamps
  include Mongoid::History::Trackable

  field :title
  field :body
  has_and_belongs_to_many :tags
  track_history on: %i[all], track_create: true, track_update: true
end

class Tag
  include Mongoid::Document

  field :title
  has_and_belongs_to_many :posts
end

Create with tags works as expected:

tag = Tag.create!
post = Post.create!(tags: [tag])
post.history_trackers.to_a
=> [#<Tracker _id: 5a579e2e017491024889a7b9, 
  created_at: 2018-01-11 17:26:06 UTC, 
  updated_at: 2018-01-11 17:26:06 UTC, 
  association_chain: [{"name"=>"Post", "id"=>BSON::ObjectId('5a579e2e017491024889a7b8')}], 
  modified: {"tag_ids"=>[BSON::ObjectId('5a579e2e017491024889a7b7')]}, 
  original: {}, 
  version: 1, 
  action: "create", 
  scope: "post", 
  modifier_id: nil>]

However, changes are not recorded:

tag = Tag.create!
tag2 = Tag.create!
post = Post.create!(tags: [tag])
post.tags << tag2
post.history_trackers.to_a
=> [#<Tracker _id: 5a579e2e017491024889a7b9, 
  created_at: 2018-01-11 17:26:06 UTC, 
  updated_at: 2018-01-11 17:26:06 UTC, 
  association_chain: [{"name"=>"Post", "id"=>BSON::ObjectId('5a579e2e017491024889a7b8')}], 
  modified: {"tag_ids"=>[BSON::ObjectId('5a579e2e017491024889a7b7')]}, 
  original: {}, 
  version: 1, 
  action: "create", 
  scope: "post", 
  modifier_id: nil>]

Unfortunately, none of the document callbacks (i.e. around_update, around_create, around_destroy) are called when modifications are made to a has_and_belongs_to_many relation. From my reading of the Mongoid docs and code, the only way to be notified of changes to these relations is to attach a callback directly to the relation definition.

While inconvenient, this proposed solution simply makes a callback method available that must be attached to each has_and_belongs_to_many relation via before_add: :track_has_and_belongs_to_many, before_remove: :track_has_and_belongs_to_many. I don't know of a way to automatically attach these callbacks, but please educate me if this is possible.

Here's an example of this solution in use:

class Post
  include Mongoid::Document
  include Mongoid::Timestamps
  include Mongoid::History::Trackable

  field :title
  field :body
  has_and_belongs_to_many :tags, before_add: :track_has_and_belongs_to_many, before_remove: :track_has_and_belongs_to_many
  track_history on: %i[all], track_create: true, track_update: true
end

class Tag
  include Mongoid::Document

  field :title
  has_and_belongs_to_many :posts
end

tag = Tag.create!
tag2 = Tag.create!
post = Post.create!(tags: [tag])
post.tags << tag2
post.history_trackers.to_a
=> [
  #<Tracker _id: 5a57aa610174912e0c3edf88, 
    created_at: 2018-01-11 18:18:09 UTC, 
    updated_at: 2018-01-11 18:18:09 UTC, 
    association_chain: [{"name"=>"Post", "id"=>BSON::ObjectId('5a57aa610174912e0c3edf87')}], 
    modified: {"tag_ids"=>[BSON::ObjectId('5a57aa610174912e0c3edf85')]}, 
    original: {}, 
    version: 1, 
    action: "create", 
    scope: "post", 
    modifier_id: nil>, 
  #<Tracker _id: 5a57aa610174912e0c3edf89, 
    created_at: 2018-01-11 18:18:09 UTC, 
    updated_at: 2018-01-11 18:18:09 UTC, 
    association_chain: [{"name"=>"Post", "id"=>BSON::ObjectId('5a57aa610174912e0c3edf87')}], 
    modified: {"tag_ids"=>[BSON::ObjectId('5a57aa610174912e0c3edf85'), BSON::ObjectId('5a57aa610174912e0c3edf86')]}, 
    original: {"tag_ids"=>[BSON::ObjectId('5a57aa610174912e0c3edf85')]}, 
    version: 2, 
    action: "update", 
    scope: "post", 
    modifier_id: nil>
]

[mongoid-bot]

	1 Warning
⚠️	Unless you’re refactoring existing code, please update CHANGELOG.md.

Here's an example of a CHANGELOG.md entry:

* [#215](https://github.com/mongoid/mongoid-history/pull/215): Solution for tracking has_and_belongs_to_many relations - [@mikwat](https://github.com/mikwat).

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[coveralls]

Coverage increased (+0.006%) to 99.809% when pulling 93acc0c on mikwat:has-and-belongs-to-many into bc5a0fa on mongoid:master.

[mikwat]

Carryover of conversation from #214:

@dblock: I think we should be able to attach those callbacks by examining all HABTM relationships in late binding, similarly to how we examine tracked fields. This would avoid the whole explicit options to the relationship, wouldn't it?

@mikwat: That would be great, but I'm not sure it's possible. reflect_on_all_associations(:has_and_belongs_to_many) returns an array of Mongoid::Relations::Metadata instances, but how would you use these to add the necessary callbacks on the relation?

@dblock: Maybe @estolfo can help with some pointers, please?

[dblock]

I've tried to build something by following the embeds model. I can get to the association, but don't know how to insert the callback from the association. WIP in dblock@1209422.

[mikwat]

I like the direction you're going with this @dblock, but I think we're running into the same problem.

[dblock]

So we can add the callback by overwriting has_and_belongs_to_many, but I am not sure we should. See #217. Thoughts? We can leave that one open and carry over the conversation since it passes tests and such.

[johnnyshields]

We should only modify the callbacks in cases where the habtm attributes have been explicitly whitelisted.

[mikwat]

See #217