Cannot set payment method Purchase Order on cart

[galaoleksandr]

Preconditions (*)

1. Customer exists with e-mail EMAIL and password PASS
2. At least one product exists
3. Purchase order payment method is enabled

Steps to reproduce (*)

1. Generate customer token using GraphQl:

   mutation {
   generateCustomerToken(
   email:  {{ EMAIL }}
   password:  {{ PASS }}
   ) {
   token
   }
   }

Response:

{
  "data": {
    "generateCustomerToken": {
      "token":  {{ TOKEN }}
    }
  }
}

2. Create empty cart:

   mutation {
   createEmptyCart
   }

Headers:

{
"Content-Type": "application/json",
"Authorization": "Bearer {{  TOKEN  }}"
}

Response:

{
  "data": {
    "createEmptyCart": {{  CART_ID  }}
  }
}

3. Login to storefront using EMAIL and PASS.
4. Add product to cart on storefront.
5. Go to checkout.
6. Set shipping method.
7. Try to set payment method Purchase Order using GraphQl:

mutation { setPaymentMethodOnCart (
  input: {
    cart_id: {{  CART_ID  }}
    payment_method: {
      code: "purchaseorder"
      purchase_order_number: "12345"
    }
  })
 {
   cart {

      available_payment_methods {
        code
        title
      }
      }
    }
} 

Headers:

{
"Content-Type": "application/json",
"Authorization": "Bearer {{ TOKEN  }}"
}

Expected result (*)

Purchase order is set

Actual result (*)

Response:

{
  "errors": [
    {
      "message": "Purchase order number is a required field.",
      "category": "graphql-input",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": [
        "setPaymentMethodOnCart"
      ]
    }
  ],
  "data": {
    "setPaymentMethodOnCart": null
  }
}

[naydav]

https://github.com/magento/graphql-ce/pull/578