Rails models cheatsheet

items = Model
  .where(first_name: 'Harvey')
  .where('id = 3')
  .where('id = ?', 3)

  .order(:title)
  .order(title: :desc)
  .order("title DESC")

  .reorder(:title)  # discards other .order's
  .rewhere(...)     # discards other .where's

  .limit(2)
  .offset(1)
  .uniq

See: QueryMethods

items = Model
  .select(:id)
  .select([:id, :name])

  .group(:name)   # GROUP BY name
  .group('name AS grouped_name, age')
  .having('SUM(price) > 30')  # needs to be chained with .group

  .includes(:user)
  .includes(user: [:articles])

  .references(:posts)
  # aka: .where("posts.name = 'foo'").references(:posts)

item = Model.find(id)
item = Model.find_by_email(email)
item = Model.where(email: email).first

Model
  .exists?(5)
  .exists?(name: "David")

  .first
  .last
  .find_nth(4, [offset])

See: FinderMethods

item.new_record?
item.persisted?
item.destroyed?

item.serialize_hash

item.save
item.save!      # Same as above, but raises an Exception

item.update  name: 'John'  # Saves immediately
item.update! name: 'John'

item.update_column  :name, 'John'  # skips validations and callbacks
item.update_columns  name: 'John'
item.update_columns! name: 'John'

item.touch                 # updates :updated_at
item.touch :published_at

item.destroy
item.delete  # skips callbacks

Model.create     # Same an #new then #save
Model.create!    # Same as above, but raises an Exception

See: Persistence

item.attributes                      # #<Hash>

item.attributes = { name: 'John' }   # Merges attributes in. Doesn't save.
item.assign_attributes name: 'John'  # Same as above

See: AttributeAssignment

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')

See: Dirty

item.valid?
item.invalid?

See: Validations

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

See: Calculations

Given a field called name:

# Returns one record
Person.find_by_name(name)
Person.find_last_by_name(name)
Person.find_or_create_by_name(name)
Person.find_or_initialize_by_name(name)

# Returns a list of records
Person.find_all_by_name(name)

# Add a bang to make it raise an exception
Person.find_by_name!(name)

# You may use `scoped` instead of `find`
Person.scoped_by_user_name