Scoped and scope in rails

Question

Can somebody explain what this method does and what I can pass to it?

scoped(options = nil)
Returns an anonymous scope.

And also what the scope method does? I don't understand after reading the documentation.

Answer 1

In ActiveRecord, all query building methods (like where, order, joins, limit and so forth) return a so called scope. Only when you call a kicker method like all or first the built-up query is executed and the results from the database are returned.

The scoped class method also returns a scope. The scope returned is by default empty meaning the result set would not be restricted in any way meaning all records would be returned if the query was executed. You can use it to provide an "empty" alternative like in the query_by_date example by MurifoX. Or you can use it to combine multiple conditions into one method call, like for example:

Model.scoped(:conditions => 'id < 100', :limit => 10, :order => 'title ASC')

# which would be equivalent to
Model.where('id < 100').limit(10).order('title ASC')

The scope class method allows you to define a class method that also returns a scope, like for example:

class Model
  scope :colored, lambda {|col|
    where(:color => col)
  }
end

which can be used like this:

Model.colored

The nice thing with scopes is that you can combine them (almost) as you wish, so the following is absolutely possible:

Model.red.where('id < 100').order('title ASC').scoped(:limit => 10)

I also strongly suggest reading through http://guides.rubyonrails.org/active_record_querying.html

Answer 2

I have used it in the past.When you make chained calls to the ActiveRecord query interface like this:

Model.where(:conditions).where(:more_conditions).where(:final_conditions)

Each one of them is already scoped, making the chain work without any problems. But let's say you have something like this:

Model.query_by_date(date).query_by_user(user).query_by_status(status)

scope :query_by_date, lambda { |date|
  case date
  when "today"
    where(:date => Date.today)
  when "tomorrow"
    where(:date => Date.tomorrow)
  else
    # Any value like '' or 0 or Date.whatever
  end
}

This would cause an error if the date param is not today or tomorrow. It would pick the last value and try to chain this query with the next one query_by_user, resulting in a undefined method default_scoped? for ''. But if you put a scoped method in the else condition, it would work without any flaws, because you are saying to activerecord that you pass through this method/named scope and didn't make any calls to where/find/other activerecord methods, but returned a scoped object, so you can continue chaining queries and stuff.
It would be this way in the end.

else
  scoped
end

Hope you understand this simple example.