Question

I am trying to do a safe insert using GORM for Mongo’s low-level API.

I have reproduced the problem in a clean Grails project like follows:

1. Create a new Grails project
2. Uninstall the Hibernate plugin
3. Install the GORM for Mongo plugin

4. Create a controller with the following action

   import com.mongodb.*
   
   class TestController {
   
       def mongo
   
       def index = {
   
           def database = mongo.getDB("ExampleDatabase")
           def collection = database.getCollection("ExampleCollection")
   
           def document = new BasicDBObject();
           document.put("key", "value")
   
           collection.insert(document, WriteConcern.SAFE)
   
           render ""
   
       }
   }
   

5. When firing the action, the following exception is thrown:

   2011-07-27 12:53:03,161 [http-8080-1] ERROR errors.GrailsExceptionResolver  - Exception occurred when processing request: [GET] /WriteConcern.SAFE-test/test/index
   Stacktrace follows:
   groovy.lang.MissingPropertyException: No such property: value for class: com.mongodb.WriteConcern
       at com.gmongo.internal.Patcher$__converAllCharSeqToString_closure2.doCall(Patcher.groovy:81)
       at com.gmongo.internal.Patcher._converAllCharSeqToString(Patcher.groovy:80)
       at com.gmongo.internal.Patcher$_converAllCharSeqToString.callStatic(Unknown Source)
       at com.gmongo.internal.Patcher$_converAllCharSeqToString.callStatic(Unknown Source)
       at com.gmongo.internal.Patcher._convert(Patcher.groovy:69)
       at com.gmongo.internal.Patcher$_convert.callStatic(Unknown Source)
       at com.gmongo.internal.Patcher$__patchInternal_closure1.doCall(Patcher.groovy:31)
       at writeconcern.safe.test.TestController$_closure1.doCall(TestController.groovy:17)
       at writeconcern.safe.test.TestController$_closure1.doCall(TestController.groovy)
       at java.lang.Thread.run(Thread.java:680)
   

6. If I change the action to use the Mongo Java API as follows:

   def index = {
   
       def database = new Mongo().getDB("ExampleDatabase")
       def collection = database.getCollection("ExampleCollection")
   
       def document = new BasicDBObject();
       document.put("key", "value")
   
       collection.insert(document, WriteConcern.SAFE)
   
       render ""
   
   }
   

7. Now it works and the document is persisted to the Mongo database as expected.

My question is this: Is this a bug with the GMongo wrapper, or then how should safe writes be done using the low level API?

Answer 1

This appears due to the GMongo library and how it patches the DBCollection object to handle passing of Map objects to the insert method and converts them. It assumes that all of the arguments of the insert method are Map objects and will then try to get the value property from the Map.Entry.

Looking at the source of Patcher.groovy from GMongo library you’ll see the function _convert() that attempts to do this. It looks like a fork of the Github project with type check on the argument (either to see if it’s a WriteConcern or to check if it’s actually a Map before passing to the _converAllCharSeqToString) is necessary.

EDIT:

I created a pull request on Github for the appropriate code change, but as with all things Groovy, patching the class can also help. You can “patch” the WriteConcern class in your BootStrap.groovy to have a getValue method and that will allow you to pass the parameter in:

def init = { servletContext ->
    com.mongodb.WriteConcern.metaClass.getValue  = { null }
}