validation error through FactoryGirl

I have problems on validating my data through FactoryGirl.

For team.rb, the custom validations are has_only_one_leader and belongs_to_same_department

class Team < ActiveRecord::Base
    attr_accessible :name, :department, :active

    has_many :memberships
    has_many :users, through: :memberships

    accepts_nested_attributes_for :memberships, :users

    validates :department, inclusion: [nil, "Architectural", "Interior Design"]
    validates_uniqueness_of :name, scope: :department, conditions: -> { where(active: true) }
    validate :has_only_one_leader
    validate :belongs_to_same_department

    def has_only_one_leader
        unless self.users.where!(designation: "Team Leader").size == 1
            errors.add(:team, "needs to have exactly one leader")
        end
    end

    def belongs_to_same_department
        unless self.users.where!.not(department: self.department).size == 0
            errors.add(:users, "should belong to the same department")
    end
end

I'll also include membership.rb and user.rb (associations only) just for reference.

class Membership < ActiveRecord::Base
    belongs_to :team
    belongs_to :user
end

class User < ActiveRecord::Base
    has_many :memberships
    has_many :teams, through: :memberships
end

Here's my factory for team.rb

FactoryGirl.define do
    factory :team do
        sequence(:name) {|n| "Team #{n}" }
        department "Architectural"

        before(:create) do |team|
            team.users << FactoryGirl.create(:user, designation: "Team Leader",
                department: "Architectural")
            team.users << FactoryGirl.create_list(:user, 5, 
                designation: "CAD Operator", department: "Architectural")
        end
    end
end

It seems that after I do FactoryGirl.create(:team) in the rails console, it seems that I got the error messages from my validations.

Two things I've noticed when I manually build a team, specifically adding members to the team with 1 leader and 5 members belonging to the same department:

team.users.where!(designation: "Team Leader").size returns 6 although there's only one leader, same goes for changing the designation to CAD Operator, returns 6 although there are only five non leaders in the team.
same goes for checking if the members are in the same department, team.users.where!.not(department: team.department).size returns 6, although all of them belong to the same department.

Are there any modifications to make in my model or in my factory?

Here are the things that I've discovered before getting the answer.

team.users.where(designation: "Team Leader") returns a User::ActiveRelation_AssociationRelation object
team.users returns a User::ActiveRecord_Associations_CollectionProxy object
CollectionProxy has no method where since the this method (in my opinion) is a query to the database (with exception if the team is already saved in the database, you can use where, but in this case, it's only for building)

Therefore, I used the select method accompanied with the count, to return the correct values like so. I'll use my example from the question to illustrate the answer.

team.users.select(:designation) {|user| user.designation == "Team Leader"}.count returns 1
team.users.select(:department) {|user| user.department != team.department}.count returns 0