I need to create a many-to-many relationship in Grails.
I have a "Question" domain and a "Tag" domain.
A Question can have 0 or more tags. A Tag can have 0 or more Questions.
If I put a "hasMany" on each sides, it gives me an error saying I need a "belongTo" somewhere.
However, adding a belongsTo means that the owner must exist...
Like I said, a Tag could have 0 questions, and a Question could have 0 tags.
There is no concept of an owner, it's a many-to-many!
What am I supposed to do?
you can do this (please see the code below). but does it make sense to have a question tag with out both a question and a tag?
package m2msansbt
class Question {
String toString() { return name }
String name
static hasMany=[questionTags:QuestionTag]
static constraints = {
}
}
package m2msansbt
class Tag {
String toString() { return name }
String name
static hasMany=[questionTags:QuestionTag]
static constraints = {
}
}
package m2msansbt
class QuestionTag {
Question question
Tag tag
static QuestionTag link(Question question,Tag tag) {
QuestionTag questionTag=QuestionTag.findByQuestionAndTag(question,tag)
if (!questionTag) {
questionTag = new QuestionTag()
question?.addToQuestionTags(questionTag)
tag?.addToQuestionTags(questionTag)
questionTag.save()
}
return questionTag
}
static void unlink(Question question,Tag tag) {
QuestionTag questionTag=QuestionTag.findByQuestionAndTag(question,tag)
if (questionTag) {
question?.removeFromQuestionTags(questionTag)
tag?.removeFromQuestionTags(questionTag)
questionTag.delete()
}
}
static constraints = {
}
}
import m2msansbt.*
class BootStrap {
def init = { servletContext ->
Question q1=new Question(name:'q1')
Tag t1=new Tag(name:'t1')
Tag t2=new Tag(name:'t2')
q1.save()
t1.save()
t2.save()
QuestionTag q1t1=QuestionTag.link(q1,t1)
q1t1.save()
QuestionTag q1t2=QuestionTag.link(q1,t2)
q1t2.save()
q1.save()
t1.save()
}
def destroy = {
}
}
If your main concern is the cascading delete, you can take a look at 5.5.2.9 in the grails docs to manually disable it for the mapping.
I haven't tried it, but I think the mappedBy property can be used to solve this.
Checkout the Taggable Plugin.
It seems to solve the problem you are having and you could look at the source code if you want to see how they modeled the relationship. BTW this plugin was originally created by Graeme Rocher who is the lead developer on Grails.
This works for me on Grails 2.4.4. Add a "belongsTo" with just the classname.
class Question {
String toString() { return name }
String name
static hasMany=[tags:Tag]
static constraints = {
}
}
class Tag {
String toString() { return name }
String name
static hasMany=[questions:Question]
static belongsTo = Question
static constraints = {
}
}
Related
I have a grails domain class with an embedded object, I want to validate the embedded object's attributes only while updating.
I know I can do this on a normal grails domain class by using a custom validator and checking if the domain's class id is not null.
I can't do this because of the lack of an id on an embedded object.
There is a little example of what I want to do.
//This is on domain/somePackage/A.groovy
class A{
B embeddedObject
static embedded = ['embeddedObject']
static constraints = {
embeddedObject.attribute validator:{val, obj-> //The app fails to start when this code is added!!
if(obj.id && !val) //if the id is not null it means the object it's updating...
return 'some.error.code'
}
}
}
//this class is on src/groovy/somePackage/B.groovy
class B{
String attribute
static constraints={
attribute validator:{val,obj->
if(obj.id && !val) //This will fail too! because the lack of an id on this object....
return 'some.error.code'
}
}
}
Is there a way to get the id of the 'parent' on the embedded object??
Any help will be appreciated
way too complicated:
class A{
B embeddedObject
static embedded = ['embeddedObject']
static constraints = {
embeddedObject validator:{ val, obj ->
if( obj.id && !val.attribute )
return 'some.error.code'
}
}
}
I'm having a problem where the related table id fields return 'null' from my domain objects when using inheritance. Here is an example:
In /src/groovy/
BaseClass1.groovy
class BaseClass1 {
Long id
static mapping = {
tablePerConcreteClass true
}
}
BaseClass2.groovy
class BaseClass2 extends BaseClass1 {
String someOtherProperty
static constraints = {
someOtherProperty(maxSize:200)
}
static mapping = BaseClass1.mapping
}
In /grails-app/domain
ParentClass.groovy
class ParentClass extends BaseClass2 {
ChildClass myChild
static mapping = BaseClass2.mapping << {
version false
}
}
ChildClass.groovy
class ChildClass extends BaseClass1 {
String property
static mapping = BaseClass1.mapping
}
The problem appears here:
SomeotherCode.groovy
print parentClassInstance.myChild.id // returns the value
print parentClassInstance.myChildId // returns null
Any ideas what might be going on to get those dynamic properties to break like this?
After debugging into the get(AssociationName)Id source, I found the following:
The handler for this is:
GrailsDomainConfigurationUtil.getAssociationIdentifier(Object target, String propertyName,
GrailsDomainClass referencedDomainClass) {
String getterName = GrailsClassUtils.getGetterName(propertyName);
try {
Method m = target.getClass().getMethod(getterName, EMPTY_CLASS_ARRAY);
Object value = m.invoke(target);
if (value != null && referencedDomainClass != null) {
String identifierGetter = GrailsClassUtils.getGetterName(referencedDomainClass.getIdentifier().getName());
m = value.getClass().getDeclaredMethod(identifierGetter, EMPTY_CLASS_ARRAY);
return (Serializable)m.invoke(value);
}
}
catch (NoSuchMethodException e) {
// ignore
}
catch (IllegalAccessException e) {
// ignore
}
catch (InvocationTargetException e) {
// ignore
}
return null;
}
It threw an exception on the related class (value.getClass().getDeclaredMethod), saying NoSuchMethod for the method getId(). I was unable to remove the id declaration from the base class without Grails complaining that an identifier column was required. I tried marking id as public and it also complained that it wasn't there. So, I tried this
BaseClass {
Long id
public Long getId() { return this.#id }
}
and things worked on some classes, but not on others.
When I removed the ID declaration, I go an error: "Identity property not found, but required in domain class". On a whim, I tried adding #Entity to the concrete classes and viola! everything started working.
class BaseClass {
//Don't declare id!
}
#Entity
class ParentClass {}
#Entity
class ChildClass {}
I still think it is a grails bug that it needs to be added, but at least it is easy enough to work around.
I'm not sure why you are seeing this behavior, but I'm also not sure why you are doing some of the things you are doing here. Why have a domain class extend a POGO? Domains, Controllers, and Services are heavily managed by the Grails machinery, which probably was not designed for this sort of use. Specifically, I believe Grails builds the dynamic property getters for the GrailsDomainProperty(s) of GrailsDomainClass(es), not POGO's. In this case, you have an explicitly declared id field in BaseClass1 that is not a GrailsDomainProperty. I suspect that this POGO id property is not picked up by the Grails machinery that creates the dynamic property getters for Domains.
You might try putting BaseClass1/2 in /grails-app/domain, perhaps making them abstract if you don't want them instantiated, then extending them as you are and seeing if you observe the behavior you want.
I have 2 domain classes
class a {
String name
static constraints = {
name unique:true
}
}
class b {
String description
}
and in domain class b I want to call class a
import a
class b {
String description
static constraints = {
description unique:'a.name'
}
}
and get error
Scope for constraint [unique] of property [description] of class [b] must be a valid property name of same class
How do I get a property from class a to b?
Assuming you try to do this in Grails 2+
You can't use validation that way. In your example you need to reference to a property of the same domain class. To correct the constraint in class B you can write:
class B {
String description
static contraints = {
description unique:true
}
}
But I think you want to import the constraints from class a which is done like this.
class B {
String description
static contraints = {
importFrom A
}
}
See http://grails.org/doc/latest/guide/validation.html#sharingConstraints
This will import all constraints on properties that the two classes share. Which in your case is none.
UPDATE
I got a similar question and found a solution for it. So I thought to share it here with you.
The problem can be solved with a custom validator. In your case constraints for class B:
static constraints = {
description(validator: {
if (!it) {
// validates to TRUE if the collection is empty
// prevents NULL exception
return true
}
def names = A.findAll()*.name
return names == names.unique()
})
}
It's difficult to answer your question correctly since the requirements are a bit odd. But maybe it will help.
You need to write a custom validator to check the uniqueness since it relies on more information than the single instance of b will have.
Something like
static constraints {
description validator: { val ->
!a.findByName(val)
}
}
might do the trick.
I'm writing some named queries for my domain classes in Grails and I've hit a blocker.
Given the following domain class:
class Contributor {
// evals is a collection of another domain class
def evals
static namedQueries = {
hasNoEvals {
// Something like this...
evals.size() == 0
}
}
}
Can anyone help with the syntax I need to select the Contributors who have no Evals?
Thanks.
Please look in createCriteria doc for "collection property" operations. In your case, it is isEmpty:
static namedQueries = {
hasNoEvals {
isEmpty('evals')
}
}
For generic size restriction, it is sizeEq, sizeLe and so on.
I have a Grails domain called People, and I want to check that each People has childs or not. Childs are other People objects. Here is my domain structure:
class People implements Serializable {
static constraints = {
name (nullable : false, unique : true)
createdBy (nullable : false)
creationDate (nullable : false)
}
static transients = ['hasChild']
static mapping = {
table 'PEOPLE'
id generator: 'sequence', params : [sequence : 'SEQ_PK_ID']
columns {
id column : 'APEOPLE_ID'
parentPeople column : 'PARENT_PEOPLE_ID'
}
parentPeople lazy : false
}
People parentPeople
String name
String description
Boolean hasChild() {
def childPeoples = People.createCriteria().count {
eq ('parentPeople', People)
}
return (childPeoples > 0)
}
}
But I cannot call people.hasChild() at anywhere. Could you please helpe me on this? Thank you so much!
It's because in eq ('parentPeople', People), Grails can't understand what "People" is (it's a class). You should replace "People" by this. For example:
static transients = ["children"]
def getChildren() {
def childPeoples = People.findAllByParentPeople(this, [sort:'id',order:'asc'])
}
Another way to get the same result is to use Named Queries. It seems more concise and was created specifically for this purpose. I also like it because it fits the pattern of static declarations in a domain model and it's essentially a criteria, which I use throughout my applications. Declaring a transient then writing a closure seems a bit of a work-around when you can declare named queries ... just my opinion.
Try something like this:
static namedQueries = {
getChildren {
projections {
count "parentPeople"
}
}
}