Question

I have two models. I am trying to query a model for how many relationships it has to the other model. My models are as follows:

# app/models/client.rb
class Client
  include Mongoid::Document
  belongs_to :contact
  ...
end

# app/models/contact.rb
class Contact
  include Mongoid::Document
  has_many :clients
  ...
end

I need to be able to query for the following:

Contacts with NO clients

Contact.where("clients.length == 0")

Contacts with Clients

Contact.where("clients.length > 0")

Can anyone help me with how I would do about this?

Answer 1

Given the following model:

class Client
  include Mongoid::Document
  belongs_to :contact
  field :name, type: String
end
class Contact
  include Mongoid::Document
  has_many :clients
  field :name, type: String
end

And the following insertions:

Contact.create(:name => "Bill")
jill = Contact.create(:name => "Jill")
jill.clients.create(:name => "Steve")

The following code will do what you need:

p "Has Clients"
Contact.any_in(_id: Client.all.distinct("contact_id")).each do |c| 
  p c 
end
p "No Clients"
Contact.not_in(_id: Client.all.distinct("contact_id")).each do |c| 
  p c 
end

Outputs:

"Has Clients"
#<Contact _id: 4f5b04b1e98c373917000002, _type: nil, name: "Jill">
"No Clients"
#<Contact _id: 4f5b04b1e98c373917000001, _type: nil, name: "Bill">

Full gist:
https://gist.github.com/2010817