Tree-Like Structure in Grails GORM

Question

I want to create a tree-like structure in Grails GORM. The structure should be a container for other objects and should full fill the following requirements:

• a node should have 0 to n childrens
• a node should have exactly one parent node
• a node should have 0 to n sibling nodes

How can I define such a structure in Grails GORM?

I tried the following class:

Class TreeNode {
    String name
    TreeNode parent

    static hasMany = [children: TreeNode]

    //returns the root node, and by extension, the entire tree!
    TreeNode getRootNode(){
       if(parent){
          //if parent is not null then by definition this node is a child node of the tree.
          return parent.getRootNode()
       }else{
          //if parent is null then by definition it is the root node.
          return this
       }
    }

    //you might not need this function, but ill add it as it is common in tree structures
    boolean isLeaf(){
       //determines if this node is a leaf node. a leaf is a node with zero childrens
       return children.isEmpty()
    }
}

But on startup I get the following error:

ERROR context.ContextLoader  - Context initialization failed
Message: Error creating bean with name 'pluginManager' defined in ServletContext resource [/WEB-INF/applicationContext.xml]: Invocation of init method failed; nested exception is org.codehaus.groovy.grails.exceptions.GrailsDomainException: Property [children] in class [class test.TreeNode] is a bidirectional one-to-many with two possible properties on the inverse side. Either name one of the properties on other side of the relationship [treeNode] or use the 'mappedBy' static to define the property that the relationship is mapped with. Example: static mappedBy = [children:'myprop']
    Line | Method
->>  334 | innerRun  in java.util.concurrent.FutureTask$Sync
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
|    166 | run       in java.util.concurrent.FutureTask
|   1145 | runWorker in java.util.concurrent.ThreadPoolExecutor
|    615 | run       in java.util.concurrent.ThreadPoolExecutor$Worker
^    722 | run . . . in java.lang.Thread
Caused by GrailsDomainException: Property [children] in class [class test.TreeNode] is a bidirectional one-to-many with two possible properties on the inverse side. Either name one of the properties on other side of the relationship [treeNode] or use the 'mappedBy' static to define the property that the relationship is mapped with. Example: static mappedBy = [children:'myprop']
->>  334 | innerRun  in java.util.concurrent.FutureTask$Sync
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
|    166 | run       in java.util.concurrent.FutureTask
|   1145 | runWorker in java.util.concurrent.ThreadPoolExecutor
|    615 | run       in java.util.concurrent.ThreadPoolExecutor$Worker
^    722 | run . . . in java.lang.Thread
| Error 2013-10-04 17:36:00,730 [localhost-startStop-1] ERROR context.GrailsContextLoader  - Error initializing the application: Error creating bean with name 'pluginManager' defined in ServletContext resource [/WEB-INF/applicationContext.xml]: Invocation of init method failed; nested exception is org.codehaus.groovy.grails.exceptions.GrailsDomainException: Property [children] in class [class test.TreeNode] is a bidirectional one-to-many with two possible properties on the inverse side. Either name one of the properties on other side of the relationship [treeNode] or use the 'mappedBy' static to define the property that the relationship is mapped with. Example: static mappedBy = [children:'myprop']
Message: Error creating bean with name 'pluginManager' defined in ServletContext resource [/WEB-INF/applicationContext.xml]: Invocation of init method failed; nested exception is org.codehaus.groovy.grails.exceptions.GrailsDomainException: Property [children] in class [class test.TreeNode] is a bidirectional one-to-many with two possible properties on the inverse side. Either name one of the properties on other side of the relationship [treeNode] or use the 'mappedBy' static to define the property that the relationship is mapped with. Example: static mappedBy = [children:'myprop']
    Line | Method
->>  334 | innerRun  in java.util.concurrent.FutureTask$Sync
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
|    166 | run       in java.util.concurrent.FutureTask
|   1145 | runWorker in java.util.concurrent.ThreadPoolExecutor
|    615 | run       in java.util.concurrent.ThreadPoolExecutor$Worker
^    722 | run . . . in java.lang.Thread
Caused by GrailsDomainException: Property [children] in class [class test.TreeNode] is a bidirectional one-to-many with two possible properties on the inverse side. Either name one of the properties on other side of the relationship [treeNode] or use the 'mappedBy' static to define the property that the relationship is mapped with. Example: static mappedBy = [children:'myprop']
->>  334 | innerRun  in java.util.concurrent.FutureTask$Sync
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
|    166 | run       in java.util.concurrent.FutureTask
|   1145 | runWorker in java.util.concurrent.ThreadPoolExecutor
|    615 | run       in java.util.concurrent.ThreadPoolExecutor$Worker
^    722 | run . . . in java.lang.Thread

Answer 1

A tree generally has one parent per node. You can define a tree node having a one-to-many relationship to itself like this:

class TreeNode {
    TreeNode parent
    static hasMany = [children: TreeNode]
    static mappedBy = [children: 'parent']
}

If it has multiple parents, it's not actually a tree in the computer science sense. This data structure is usually called a directed graph. You might be able to model it as a many to many relationship to itself, like this:

class GraphNode {
    static hasMany = [children: GraphNode]
    static hasMany = [parents: GraphNode]
    static mappedBy = [children: 'parents', parents: 'children']
}