mongoid embeds_many

mem.rb

class A
  include Mongoid::Document
  embeds_many :items
end

class Item
  include Mongoid::Document
  embedded_in :a
  field :text, type: String
end

a = A.new(items: [])
a.save
# > db.as.find({_id:ObjectId("5b75482054ffa1008b98cca2")})
# {
#     "_id" : ObjectId("5b75482054ffa1008b98cca2")
# }


a.items = [Item.new(text: 'unko')]
# > db.as.find({_id:ObjectId("5b75482054ffa1008b98cca2")})
# {
#     "_id" : ObjectId("5b75482054ffa1008b98cca2"),
#     "items" : [ 
#         {
#             "_id" : ObjectId("5b75483554ffa1008b98cca3"),
#             "text" : "unko"
#         }
#     ]
# }

a.items = []
# > db.as.find({_id:ObjectId("5b75482054ffa1008b98cca2")})
# {
#     "_id" : ObjectId("5b75482054ffa1008b98cca2"),
#     "items" : []
# }

Model#{name}=
Set the embedded documents. If the parent document is persisted, then the child will be atomically saved immediately. If setting to nil or [] then the children will be deleted.
https://mongoid.github.io/old/en/mongoid/docs/relations.html#embeds_many