Hello,

After rubocop complained, a += operator was replaced by concat on some has_and_belongs_to_many id fields.

And concat didn't work as expected.

How to reproduce :

class Blog
  include Mongoid::Document
  include Mongoid::Timestamps
  
  has_and_belongs_to_many :authors, inverse_of: nil
end

class Author
  include Mongoid::Document
  include Mongoid::Timestamps

  field :name, type: String
end

author = Author.create!(name: 'me')
Blog.create!

Blog.all.each do |blog|
  blog.author_ids.concat(Array(author.id))
  puts "first time : #{blog.author_ids}"

  blog.author_ids.concat(Array(author.id))
  puts "second time : #{blog.author_ids}"
end

Output :

first time : []
second time : [BSON::ObjectId('6278f1769d1fa2ff46467c48')]

So as author_ids is nil on the blog instance, the default value returned is an empty array.

The first time I try to concatenate something with author_ids, it seems that I finally concatenate it with the default value empty array that is not associated to the blog instance author_ids field, so author_ids is still empty.

Then, the array returned by author_ids is ok and concat will work as expected.

Note :

Using += instead of concat fixed my issue.

Any call to blog.author_ids before concat also fix the issue.