Scopes are partial query conditions to database queries. scopes are always prefixed to class finders. There are several ways to use scopes inside rails models.
# 1. Scope defined below gets loaded while class definition loads
scope :active, where(:active => true)
scope :archived, where(:archived => true, :post_type => :general)
# 2.Dynamic scopes needs to be always defined inside lambda
scope :not_expired, lambda { where('expiry_date <= ?', Date.today) }
# 3.Combining scopes
scope :visible, published.not_expired
# 4. Passing parameters to scopes
# avoid below
scope :created_by_user, lambda {|user|
where('user_id = ?', user)
}
# use this
scope :created_by_user, lambda {|user|
where(:user_id => user)
}
# 5. passing multiple parameters
# avoid below
scope :made_between, lambda{|from, to|
where('created_date >= ? and created_date <= ?', from, to)
}
# use this
scope :made_between, lambda{|from, to|
where('created_date >= :from and created_date <= :to', :from => from, :to => to)
}
# 6. associations inside scope (joins and includes)
# below will perform eager loading effective when rendering posts with comments
scope :with_user_comments, lambda{|user|
includes(:comments).where('comments.user_id = ?', user)
}
# faster
# also can be done as post.comments.where(:user_id => user)
scope :with_user_comments, lambda{|user|
joins(:comments).where('comments.user_id = ?', user)
}
So, at last would suggest making use of symbols when there are multiple parameters to scopes and make maximum use of scopes rather than having where conditions everywhere :)
