class Post
  include Mongoid::Document

  embeds_many :company_tags
  embeds_many :user_tags
end


class CompanyTag
  include Mongoid::Document

  field :title, type: String

  embedded_in :post
end


class UserTag
  include Mongoid::Document

  field :title, type: String

  embedded_in :post
end

post = Post.create
post.assign_attributes(company_tags: [{id: BSON::ObjectId.new, title: 'a'}],
                       user_tags: [{id: BSON::ObjectId.new, title: 'b'}])
post.save
post.reload
p post.company_tags # [#<CompanyTag .., title: "a">]
p post.user_tags # [#<UserTag .., title: "b">] 

post.assign_attributes(company_tags: [{id: BSON::ObjectId.new, title: 'c'}],
                       user_tags: [])
post.save
p post.company_tags # [#<CompanyTag .., title: "c">] CORRECT
p post.user_tags # [] CORRECT
post.reload
p post.company_tags # [#<CompanyTag .., title: "c">] CORRECT
p post.user_tags # [] CORRECT 

post.assign_attributes(company_tags: [{id: BSON::ObjectId.new, title: 'c'}],
                       user_tags: [])
post.save
p post.company_tags # [#<CompanyTag .., title: "c">] CORRECT
p post.user_tags # [] CORRECT
post.reload
p post.company_tags # [#<CompanyTag .., title: "a">, #<CompanyTag .., title: "c">] WRONG
p post.user_tags # [] CORRECT 

# 7.3.3
# "u"=>{"$unset"=>{"user_tags"=>true}, "$set"=>{"company_tags"=>[{"_id"=>BSON::ObjectId('627d54d358af14bf60afa0b9'), "title"=>"c"}]}}

# 7.3.4
# "u"=>{"$unset"=>{"user_tags"=>true}, "$push"=>{"company_tags"=>{"$each"=>[{"_id"=>BSON::ObjectId('627d555558af14c2327e7592'), "title"=>"c"}]}}}