CODEWITHSUNDEEP
rubymongodbsinatramongoid
Alan Coromano
Alan Coromano

Reputation: 26048

Scopes and the methods those do the same thing

I want to define 2 methods in Mongoid: expensive? and the scope for it. Here is what I'm doing:

class MyItem
  include Mongoid::Document
  include Mongoid::Timestamps

  # it could find expensive and cheap items depending of is_expensive parameter
  scope :expensive, ->(is_expensive = true) do
    if is_expensive
      where(:expensive?)
    else
      not_in(:expensive?)
    end
  end

  def expensive?
    price >= 10 # $10
  end
end

So I want to able to find items the following ways:

  MyItem.expensive #find all expensive ones
  MyItem.where(:expensive?) #the same as above
  MyItem.first.expensive? #check if it's expensive
  items.expensive # items is the collection of MyItem

They don't work. For instance, MyItem.where(:expensive?) says undefined method each_pair for :expensive?:Symbol

Especially I want to figure out how to do the method or scope which will work as an instance method (not a class method) - items.expensive

Upvotes: 0

Views: 85

Answers (1)

Casper
Casper

Reputation: 34328

I don't have experience with Mongoid, so the answer might not be fully correct, but to me it seems you are mixing two things: database querying, and calling methods on an instance.

They are two separate things and can not be intermixed. For example:

def expensive? 
  price >= 10
end

...is not a database query method. You're simply looking at an instance variable and checking if it's >= 10. Therefore you can't use this method in database queries since it's not a query construct.

But to make it work should not be too hard. All you need to change is this:

scope :expensive ... do
  is_expensive ? where(:price.gte => 10) : where(:price.lt => 10)
end

The argument to where must always be a Mongoid query expression. It cannot be something else.

Now your setup should work. You can ask everything you need:

MyItem.expensive          # Returns a collection of expensive items
MyItem.first.expensive?   # Calls the instance method. Returns true or false
items.expensive           # Returns all expensive items in the collection

However this is will not work:

MyItem.where(:expensive?)

Because :expensive? is not a valid query expression, which is what is always needed as an argument to where.

Upvotes: 3

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