add some final notes for tomorrows lecture

Author: Saicheg

notes/9.md

@@ -71,11 +71,13 @@ end
 
 ```
 User.all
-User.first
+User.first[()]
+User.take[()]
 User.find(2)
 User.find_by_age(18) # deprecated
 User.find_by(age: 18)
 User.find_or_create_by(email: 'ololol@mail.ru')
+User.find_each / find_in_batches
 
 User.limit(10)
 User.offset(10)
@@ -91,10 +93,13 @@ scope.reorder(:email).all
 User.limit(4).to_sql
 User.limit(4).unscoped.to_sql
 User.limit(4).unscoped.none.to_sql
+User.limit(4).ids
 
 ```
 
+
 ```
+User.order(age: :desc)
 User.order(:age).reverse_order
 User.limit(5).select(:name)
 User.limit(5).pluck(:name)
@@ -109,8 +114,10 @@ Hash[User.group(:age, :id).having('age > 18').select('COUNT(*) as number, age as
 
 ```
 User.where(age: 18)
+User.where.not(age:18)
 User.where(age: 18).where(password: '1234')
 User.where(age: 18, email: 'lol@gmail.com')
+User.where(age: 18).exists?
 age=18; User.where('age >= #{age}');
 User.where('age >= ?', 18)
 Usere.where('age >= ? OR email ILIKE ?, 18, 'test@gmail.com')
@@ -159,19 +166,99 @@ end
 
 ```
 
+## Scopes
+
+```
+class User < ActiveRecord::Base
+  default_scope { where("removed_at IS NULL") }
+  scope :created_before, ->(time) { where("created_at < ?", time) }
+end
+
+```
+
+## Callbacks
+http://guides.rubyonrails.org/active_record_callbacks.html
+
+## Creating an Object
+* before_validation
+* after_validation
+* before_save
+* around_save
+* before_create
+* around_create
+* after_create
+* after_save
+
+### Updating an Object
+* before_validation
+* after_validation
+* before_save
+* around_save
+* before_update
+* around_update
+* after_update
+* after_save
+
+### Destroying an Object
+* before_destroy
+* around_destroy
+* after_destroy
+
 
 ## Relations
 
+* http://guides.rubyonrails.org/association_basics.html
+* belongs_to
+* has_one
+* has_many
+* has_many throught: 
 * (table_name)_count - Used to cac* he the number of belonging objects on associations.
+* people vs profile
+* slavers vs people
+* people vs toilets
+
 
-## STI
 
 ## Polymorphic associations
 (association_name)_type - Stores the type for polymorphic associations.
 
+```
+class Picture < ActiveRecord::Base
+  belongs_to :imageable, polymorphic: true
+end
+ 
+class Employee < ActiveRecord::Base
+  has_many :pictures, as: :imageable
+end
+ 
+class Product < ActiveRecord::Base
+  has_many :pictures, as: :imageable
+end
+
+class CreatePictures < ActiveRecord::Migration
+  def change
+    create_table :pictures do |t|
+      t.string  :name
+      t.integer :imageable_id
+      t.string  :imageable_type
+      t.timestamp
+      OR
+      t.references :imageable, polymorphic: true
+    end
+  end
+end
+
+
+```
+
+## STI
+
+class Admin < User
+end
 
 ## Serialization
 
+
 ## Optimistic lock
 
 lock_version - Adds optimistic locking to a model.