ActiveRecord: has_many relationship returns Array instead of ActiveRecord::Relation

I have the following code:

class User < ActiveRecord::Base
  has_many :transactions
end

class Transaction < ActiveRecord::Base
  belongs_to :user
  belongs_to :owner, class_name: 'User'

  scope :user, ->(user) { where('user_id = ?', user.id) }
  scope :owner, ->(user) { where('owner_id = ?', user.id) }
  scope :active, where(is_active: true)
end

If I type the following in the console I have:

Transaction.user(User.first).class # ActiveRecord::Transaction OK
User.first.transactions.class # Array KO
User.first.transactions.active.class # ActiveRecord::Transaction OK
User.first.transactions.where(used_id: User.first.id).class # ActiveRecord::Transaction OK
User.first.transactions.owner(User.first) # ERROR: 

NoMethodError: undefined method `+' for #<User:0x00000007d7d528>
    from /home/augustin/.rvm/gems/ruby-2.1.4/gems/activemodel-3.2.21/lib/active_model/attribute_methods.rb:407:in `method_missing'
    from /home/augustin/.rvm/gems/ruby-2.1.4/gems/activerecord-3.2.21/lib/active_record/attribute_methods.rb:149:in `method_missing'
    from /home/augustin/.rvm/gems/ruby-2.1.4/gems/activesupport-3.2.21/lib/active_support/core_ext/array/access.rb:19:in `to'
    from /home/augustin/.rvm/gems/ruby-2.1.4/gems/activerecord-3.2.21/lib/active_record/associations/collection_proxy.rb:91:in `method_missing'
    from (irb):14
    from /home/augustin/.rvm/gems/ruby-2.1.4/gems/railties-3.2.21/lib/rails/commands/console.rb:47:in `start'
    from /home/augustin/.rvm/gems/ruby-2.1.4/gems/railties-3.2.21/lib/rails/commands/console.rb:8:in `start'
    from /home/augustin/.rvm/gems/ruby-2.1.4/gems/railties-3.2.21/lib/rails/commands.rb:41:in `<top (required)>'
    from script/rails:40:in `require'
    from script/rails:40:in `<main>'

And the craziest thing is that:

User.first.transactions.active.owner(User.first)

Works!!

I'm using Rails 3.2 so pretty old I know, but still, annoying, right?

Questions are:

• Shouldn't User.first.transactions.class be an ActiveRecord::Relation?
• Why does the error occurs at User.first.transactions.owner(User.first)? Is it an ActiveRecord bug or am I doing something wrong?