Persisting Maps and Lists of properties as JSON in

问题:

EDIT: onload() method changed to afterLoad(): Otherwise objects might not be passed properly to the map.

---

I am currently using some domain classes with a lot of dynamic, complex properties, that I need to persist and update regularly.

I keep these in a Map structure for each class since this makes it easy for referencing in my controllers etc.

However, since Grails does not seem to be able to persist complex property types like List and Map in the DB I am using the following approach to achieve this via JSON String objects:

class ClassWithComplexProperties {

  Map complexMapStructure //not persisted
  String complexMapStructureAsJSON //updated and synched with map via onload,beforeInsert,beforeUpdate


  static transients = ['complexMapStructure']

  def afterLoad() {  //was previously (wrong!): def onLoad() {
    complexMapStructure=JSON.parse(complexMapStructureAsJSON)
  }
  def beforeInsert() {
    complexMapStructureAsJSON= complexMapStructure as JSON
  }
  def beforeUpdate() {
    complexMapStructureAsJSON= complexMapStructure as JSON
  }
  static constraints = {    
    complexMapStructureAsJSON( maxSize:20000)
  }
}

This works well as long I am only loading data from the DB, but I run into trouble when I want to save back my changes to the DB. E.g. when I do the following

/* 1. Load the json String, e.g. complexMapStructureAsJSON="""{
   data1:[[1,2],[3,4]],//A complex structure of nested integer lists    
   data1:[[5,6]] //Another one
    }""" :
*/
ClassWithComplexProperties c=ClassWithComplexProperties.get(1)

// 2. Change a value deep in the map: 
c.complexMapStructure.data1[0][0]=7

// 3. Try to save:

c.save(flush:true)

This will usually not work, since, I guess(?), GORM will ignore the save() request due to the fact that the map itself is transient, and no changes are found in the persisted properties.

I can make it work as intended if I hack step 3 above and change it to:

// 3.Alternative save:
complexMapStructureAsJSON="" //creating a change in persisted property (which will be overwritten anyway by the beforeUpdate closure)
c.save(flush:true)

To me this is not a very elegant handling of my problem. The questions:

1. Is there a simpler approach to persist my complex, dynamic map data?
2. If I need to do it the way I currently do, is there a way to avoid the hack in step 3 ?

回答1:

For option 2, you can use the beforeValidate event instead of beforeInsert and beforeUpdate events to ensure that the change propagates correctly.

class ClassWithComplexProperties {

  Map complexMapStructure //not persisted
  String complexMapStructureAsJSON //updated and synched with map via onload,beforeInsert,beforeUpdate


  static transients = ['complexMapStructure']

  def onLoad() {
    complexMapStructure=JSON.parse(complexMapStructureAsJSON)
  }

// >>>>>>>>>>>>>>
  def beforeValidate() {
    complexMapStructureAsJSON= complexMapStructure as JSON
  }
// >>>>>>>>>>>>>>

  static constraints = {    
    complexMapStructureAsJSON( maxSize:20000)
  }
}

回答2:

I of course do not know much about the application you are building, but it won't hurt to look up alternate data storage models particularly NOSQL databases. Grails has got some support for them too.

回答3:

Is there a simpler approach to persist my complex, dynamic map data?

Grails can persist List and Map out of the box, you don't need to write complex conversion code and abuse Json.

Example for Map:

class ClassWithComplexProperties {
    Map<String, String> properties    
}

def props = new ClassWithComplexProperties()
props.properties = ["foo" : "bar"]
props.save()

Example for List:

class ClassWithComplexProperties {
    List<String> properties
    static hasMany = [properties: String]
}

def props = new ClassWithComplexProperties()
props.properties = ["foo", "bar"]
props.save()

I think this is much easier and cleaner way how to deal with it.

回答4:

In response to

Is there a simpler approach to persist my complex, dynamic map data?

Grails can persist Sets, Lists and Maps to the database. That may be a simpler approach than dealing with JSON conversions. To have the map persisted to the database you need to include it in the hasMany property.

Map complexMapStructure
static hasMany = [complexMapStructure: dynamicComplexPropertyObject]

The documentation suggests that using a Bag may be more efficient.