todo-list.txt

belongs_to：mobile belongs_to user

    association
    association=(associate)
    build_association(attributes = {})
    create_association(attributes = {})
    create_association!(attributes = {})
    reload_association

has_one：user has_one mobile
  
    association
    association=(associate)
    build_association(attributes = {})
    create_association(attributes = {})
    create_association!(attributes = {})
    reload_association

  - user.create_mobile
  - user.build_mobile


has_many：user has_many mobiles

    collection
    collection<<(object, ...)
    collection.delete(object, ...)
    collection.destroy(object, ...)
    collection=(objects)
    collection_singular_ids
    collection_singular_ids=(ids)
    collection.clear
    collection.empty?
    collection.size
    collection.find(...)
    collection.where(...)
    collection.exists?(...)
    collection.build(attributes = {}, ...)
    collection.create(attributes = {})
    collection.create!(attributes = {})
    collection.reload

has_and_belongs_to_many

    collection
    collection<<(object, ...)
    collection.delete(object, ...)
    collection.destroy(object, ...)
    collection=(objects)
    collection_singular_ids
    collection_singular_ids=(ids)
    collection.clear
    collection.empty?
    collection.size
    collection.find(...)
    collection.where(...)
    collection.exists?(...)
    collection.build(attributes = {})
    collection.create(attributes = {})
    collection.create!(attributes = {})
    collection.reload


  - user.mobiles   =>   Collection （拥有recordset完全一样的功能）
  - user.mobiles.push(*mobiles)  # 添加mobiles
  - user.mobiles.pop(*mobiles) # 删除mobiles
  - user.mobiles = mobiles # 设置user的mobiles内容，和数据库里面的进行比对，并且该删删，该加加，该改改