rails-models: more tricks

rails-models.md

@@ -3,7 +3,7 @@ title: Models
 category: Rails
 ---
 
-### [Query methods](http://devdocs.io/rails/activerecord/querymethods#method-i-order)
+### [QueryMethods](http://devdocs.io/rails/activerecord/querymethods)
 
 ```rb
 items = Model
@@ -78,9 +78,6 @@ item.update_columns! name: "John"
 item.touch  # updates :updated_at
 item.touch :published_at
 
-item.valid?
-item.invalid?
-
 item.destroy
 item.delete  # skips callbacks
 ```
@@ -90,8 +87,50 @@ Model.create     # Same an #new then #save
 Model.create!    # Same as above, but raises an Exception
 ```
 
-### Sorting
+### [Dirty](http://devdocs.io/rails/activerecord/dirty)
 
+```rb
+item.changed?
+item.changed             # ['name']
+item.changed_attributes  # { 'name' => 'Bob' } - original values
+item.changes             # { 'name' => ['Bob', 'Robert'] }
+item.previous_changes    # available after #save
+item.restore_attributes
+
+item.name = 'Robert'
+item.name_was         # 'Bob'
+item.name_change      # [ 'Bob', 'Robert' ]
+item.name_changed?    # true
+item.name_changed?(from: 'Bob', to: 'Robert')
+```
+
+### [Validations](http://devdocs.io/rails/activerecord/validations)
+
+```rb
+item.valid?
+item.invalid?
+```
+
+### [Calculations](http://devdocs.io/rails/activerecord/calculations)
+
+```rb
+Person.count
+Person.count(:age)    # counts non-nil's
+
+Person.average(:age)
+Person.maximum(:age)
+Person.minimum(:age)
+Person.sum('2 * age')
+
+Person.calculate(:count, :all)
+```
+
+Advanced:
+
+```
+Person.distinct.count
+Person.group(:city).count
+```
 
 Generating
 ----------