Rails model that has both 'has_one' and 'has_many' but with some constraints

Question

I am mapping 2 models:

User
Account

class Account 
  has_many :users


class User
  has_one :account

The user table as the account_id in i

Answer 1

Approach 1 - Add a new association

Add an has_one association with a where lambda. This allows you to work within your current schema.

class Account 
  has_many :users
  has_one  :primary_user, -> { where(is_primary: true) }, :class_name=> "User"
end

Now:

account.users #returns all users associated with the account
account.primary_user #returns the primary user associated with the account
# creates a user with is_primary set to true
account.build_primary_user(name: 'foo bar', email: 'bar@foo.com')

Approach 2 - Add an association method

class Account 
  has_many :users do
    def primary
      where(:is_primary => true).first
    end
  end
end

Now:

account.users.primary # returns the primary account

Answer 2

It would probably be simpler to add a primary_user_id field to Account, and add a 'has_one' association for the primary_user:

class Account
  has_many :users
  has_one :primary_user, :class_name => "User"
end

class User
  has_one :account
end

If you must use the existing schema (with the :is_primary boolean flag), you can add a scope like this:

class User
  has_one :account
  scope :primary, where(:is_primary => true)
end

and then chain the scope to the users lookup:

account = Account.find(1)
primary_user = account.users.primary.first