test.rb

require 'active_record'
require 'logger'

ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: 'test.sqlite3')

ActiveRecord::Schema.define do
  create_table :users, force: true do |t|
    t.string :first_name
    t.string :last_name
    t.string :role
  end
end

class User < ActiveRecord::Base
  def self.lookup(name, *roles)
    filters = roles.map do |role|
      field = (role == 'admin') ? :first_name : :last_name
      arel_table.grouping(where(:role => role, field => name).where_values.reduce(:and))
    end.reduce(:or)
    where(filters)
  end
end

User.create [{first_name: 'Sergey', last_name: 'Avseyev', role: 'admin'},
             {first_name: 'User',   last_name: 'Sergey',  role: 'user'},
             {first_name: 'Vasily', last_name: 'Pupkin',  role: 'guest'}]

rel = User.lookup('Sergey', 'admin', 'user', 'guest')
puts rel.to_sql
puts rel.inspect

require 'irb'
silence_warnings { RAILS_DEFAULT_LOGGER = Logger.new(STDOUT) }
ActiveRecord::Base.logger = RAILS_DEFAULT_LOGGER if defined?(ActiveRecord)

IRB.start