I read that a m:m relationship often means there is a third class that isn't yet required. So I have m:m on User and Project, and I created a third domain class, ProjectMembership
The three domains are as follows (minimized for illustration purposes):
User
class User {
String name
static hasMany = [projectMemberships : ProjectMembership]
}
Project Membership
class ProjectMembership {
static constraints = {
}
static belongsTo = [user:User, project:Project]
}
Project:
class Project {
String name
static hasMany = [projectMemberships : ProjectMembership]
static constraints = {
}
}
If I have the ID of the user, how can I get a list of Project objects that they are assigned to?
There are a handful of ways - here are a couple:
def user = User.get(userId)
ProjectMembership.findAllByUser(user).collect { it.project }
or to avoid the query for the User:
ProjectMembership.withCriteria {
user {
eq('id', userId)
}
}.collect { it.project }
Be wary of queries that'll return large result sets - you'll end up with a huge in-memory list of project objects.
Related
I see there is a strategy for a one to one mapping for domain objects across different databases. But I am trying to associate two Domain objects that are in different datasources and have a one to many relationship.
class DomainA {
// default data source
}
class DomainB {
static hasmany = [domainA: DomainA]
static mapping = {
datasource 'ds2'
}
}
Any suggestions on how to make this work? Or a workaround?
Found a solution to this and it works pretty well. Solution is to create a join table in the schema you own.
E.g.
class DomainA {
// default data source
}
class DomainB {
List<DomainA> domainAList
static transients = ['domainAList']
static hasmany = [domainAIds: Integer]
static mapping = {
datasource 'ds2'
domainAIds joinTable: [name: 'DOMAINB_DOMAINA', key: 'DOMAINB_ID', column: 'DOMAINA_ID']
}
List<DomainA> getDomainAList(){
domainAList = domainAIds.collect { DomainA.get(it) }
domainAList
}
}
Is it possible to have a domain class that belongs to multiple domain classes with back reference? For instance:
class Person {
List<Book> books
static hasMany = [books: Book]
}
class Organization {
List<Books> books
static hasMany = [books: Book]
}
class Book {
def owner // what's the type?
static belongsTo = [Person, Books]
}
A Book can belong to a Person or an Organization, but not both.
Person and Organization have separate sequence IDs.
The solution I came up with is:
class Book {
Long ownerID
String ownerClass
static belongsTo = [Person, Books]
static transients = ['owner']
static constraints = {
ownerId(nullable:false, blank:false)
ownerClass(nullable:false, blank:false)
}
public BookOwner getOwner() {
grailsApplication.getArtefact("Domain", ownerClass)?.getClazz()?.get(ownerId)
}
}
where BookOwner is an Interface implemented by Person and Organization. So calling a bookInstance.owner will return a Person or Organization instance, both BookOwner.
My solution works well, but it doesn't feel right - a sure sign that I am not fully understanding what I'm doing. What's the best way to implement this? Should I completely give up on having the extremely convenient back reference?
Thank you
I guess, you should have made Owner superclass. Grails will create Owner table with field class meaning child class names (in your case: Person, Organization).
I am new to grails. I have the following requirement, how to achieve it ?
class File {
list<Employee> listOfCaseWorkersWhoHaveWorkedOnThisFile; <<<---- how to achieve this ?
static constraints = {
}
}
class Employee {
list<File> filesOwnedByHim <<<<<-------- also this ?
static constraints = {
}
}
My requirement is that a File will have a list of Employees with zero or more elements and same for Employee.
Do I have to implement one of the GORM (one-to-one, one-to-many etc) here ? If yes how ?
You can use hasMany which is one-to-many association between your classes.
class File {
static hasMany = [employees: Employee]
static constraints = {
}
}
class Employee {
static hasMany = [files: File]
static constraints = {
}
}
Then you can easily use GORM operations on it (addTo,findBy,etc).
Ref: http://grails.org/doc/latest/ref/Domain%20Classes/hasMany.html
I have 2 domain class with a many to many relationship. When I delete the entity that belongs to the other, I have to remove the relation before in order to avoid a foreign key error. These relations are connected via the third class, third table in MySQL.
class City {
String nameCity
static hasMany = [visits:Visit]
/* FIRST VARIANT. REMOVES ONE VISIT ONLY */
def beforeDelete() {
Visit.withNewSession {
def visitList = Visit.findByCity(this)
visitList.each { it.delete(flush: true) }
}
}
}
//_____________________________________________
class Visit { // it is the relation class
City city
Person person
}
//_____________________________________________
class Person {
String namePerson
static hasMany = [visits:Visit]
}
So when I delete the relation between two classes, it removes one relation only. I mean, if we have 1 City and 1 Person, and try to delete this City, the app functions OK. But if you have more than one Person attached to the City, we will have:
"Cannot delete or update a parent row: a foreign key constraint fails". But one relation is deleted.
If I try to delete the City one more time, the second Person will be deleted. My app behaves like that until the last Person to be deleted. So, beforeDelete() method works great.
My problem is I don't understand how to create a collection of relations and remove them all in a cycle (loop). If I make like this:
class City {
String nameCity
static hasMany = [visits:Visit]
/* SECOND VARIANT. TYPE CAST EXCEPTION */
Collection<Visit> visitList() {
Visit.findByCity(this)
}
def beforeDelete() {
Visit.withNewSession {
visitList().each { it.delete(flush: true) }
}
}
}
I have org.codehaus.groovy.runtime.typehandling.GroovyCastException 'Cannot cast object 'mypackage.Visit : 1' with class 'mypackage.Visit' to class 'java.util.Collection'.
Any thoughts and help highly appreciated.
Have you tried the following? In theory should work...
class City {
String nameCity
static hasMany = [visits:Visit]
class Visit { // it is the relation class
City city
Person person
static belongsTo = [city:City, person:Person]
}
class Person {
String namePerson
static hasMany = [visits:Visit]
}
And just make a normal delete. In this way, if you delete a City or a Person, all its related visits will be deleted
i have the following (simplified) domain classes
class Filter {
String name
static hasMany = [answers:Answer]
static belongsTo = [user:User]
}
class User {
String name
static hasMany = [answers:Answer, filters:Filter]
}
class Answer {
String text
}
Then i add answers to the user which is working perfectly. The problem occurs when i delete 1 answer of a user:
def delete = {
def answer = Answer.get(params.id)
def users = User.withCriteria() {
answers{
eq("id", answer.id)
}
}
for (user in users)
user.removeFromAnswers(answer)
answer.delete(flush:true)
redirect(action:"index")
}
What happens here is that ALL user --> answer associations get deleted.
I only want to delete this 1 answer and of cause all associations the answer is used.
I know this has to do with the missing belongsTo, but i can't use it because a ansswer can either belong to a user or to an filter...
You can add the belongsTo to set them to nullable:
class Answer {
String text
static belongsTo = [user:User, filter:Filter]
static constraints = {
user nullable:true
filter nullable:true
}
}
and then just delete the Answer directly in the Controller:
def delete = {
def answer = Answer.get(params.id)
answer.delete(flush:true)
}
GORM will take care of the rest the cascading for you.