title: NoMethodError (undefined method `aasm_state') when saving a record
body: <p>When saving <code>payment request</code>, there is an error popping up:</p>
  
  <pre><code>NoMethodError (undefined method `aasm_state' for #&lt;PaymentRequestx::PaymentRequest:0x007f574d229df8&gt;):
    activemodel (4.2.0) lib/active_model/attribute_methods.rb:433:in `method_missing'
    aasm (4.11.0) lib/aasm/persistence/base.rb:36:in `aasm_read_state'
    aasm (4.11.0) lib/aasm/instance_base.rb:12:in `current_state'
    aasm (4.11.0) lib/aasm/persistence/active_record_persistence.rb:233:in `aasm_invalid_state?'
    aasm (4.11.0) lib/aasm/persistence/active_record_persistence.rb:225:in `block in aasm_validate_states'
    aasm (4.11.0) lib/aasm/persistence/active_record_persistence.rb:223:in `each'
    aasm (4.11.0) lib/aasm/persistence/active_record_persistence.rb:223:in `aasm_validate_states'
    activesupport (4.2.0) lib/active_support/callbacks.rb:427:in `block in make_lambda'
    activesupport (4.2.0) lib/active_support/callbacks.rb:189:in `call'
    activesupport (4.2.0) lib/active_support/callbacks.rb:189:in `block in simple'
    activesupport (4.2.0) lib/active_support/callbacks.rb:92:in `call'
    activesupport (4.2.0) lib/active_support/callbacks.rb:92:in `_run_callbacks'
    activesupport (4.2.0) lib/active_support/callbacks.rb:734:in `_run_validate_callbacks'
    activemodel (4.2.0) lib/active_model/validations.rb:395:in `run_validations!'
    activemodel (4.2.0) lib/active_model/validations/callbacks.rb:113:in `block in run_validations!'
    activesupport (4.2.0) lib/active_support/callbacks.rb:88:in `call'
    activesupport (4.2.0) lib/active_support/callbacks.rb:88:in `_run_callbacks'
    activesupport (4.2.0) lib/active_support/callbacks.rb:734:in `_run_validation_callbacks'
    activemodel (4.2.0) lib/active_model/validations/callbacks.rb:113:in `run_validations!'
    activemodel (4.2.0) lib/active_model/validations.rb:334:in `valid?'
    activerecord (4.2.0) lib/active_record/validations.rb:58:in `valid?'
    activerecord (4.2.0) lib/active_record/validations.rb:83:in `perform_validations'
    activerecord (4.2.0) lib/active_record/validations.rb:37:in `save'
    activerecord (4.2.0) lib/active_record/attribute_methods/dirty.rb:21:in `save'
    activerecord (4.2.0) lib/active_record/transactions.rb:286:in `block (2 levels) in save'
    activerecord (4.2.0) lib/active_record/transactions.rb:347:in `block in with_transaction_returning_status'
    activerecord (4.2.0) lib/active_record/connection_adapters/abstract/database_statements.rb:213:in `block in transaction'
    activerecord (4.2.0) lib/active_record/connection_adapters/abstract/transaction.rb:188:in `within_new_transaction'
    activerecord (4.2.0) lib/active_record/connection_adapters/abstract/database_statements.rb:213:in `transaction'
    activerecord (4.2.0) lib/active_record/transactions.rb:220:in `transaction'
    activerecord (4.2.0) lib/active_record/transactions.rb:344:in `with_transaction_returning_status'
    activerecord (4.2.0) lib/active_record/transactions.rb:286:in `block in save'
    activerecord (4.2.0) lib/active_record/transactions.rb:301:in `rollback_active_record_state!'
    activerecord (4.2.0) lib/active_record/transactions.rb:285:in `save'
    /home/ubuntu/.rvm/gems/ruby-2.2.4/bundler/gems/payment_requestx-7836ca37b30f/app/controllers/payment_requestx/payment_requests_controller.rb:45:in `create'
  </code></pre>
  
  <p>The error occurs when saving in <code>payment_requests_controller.rb</code>:</p>
  
  <pre><code> if @payment_request.save
  </code></pre>
  
  <p>Before that, there is a saving of <code>payment request</code> successfully. What's the problem with gem <code>AASM</code>?</p>