I am new to Grails.
Can I use "hasOne" or "hasMany" without using "belongsTo" to another domain-class?
Thanks in advance.
Yes, you can. See examples in Grails doc: http://grails.org/doc/2.3.8/guide/GORM.html#manyToOneAndOneToOne
hasMany (without belongsTo) example from the doc:
A one-to-many relationship is when one class, example Author, has many
instances of another class, example Book. With Grails you define such
a relationship with the hasMany setting:
class Author {
static hasMany = [books: Book]
String name
}
class Book {
String title
}
In this case we have a unidirectional one-to-many. Grails will, by
default, map this kind of relationship with a join table.
hasOne (without belongsTo) example from the doc:
Example C
class Face {
static hasOne = [nose:Nose]
}
class Nose {
Face face
}
Note that using this property puts the foreign key on the inverse
table to the previous example, so in this case the foreign key column
is stored in the nose table inside a column called face_id. Also,
hasOne only works with bidirectional relationships.
Finally, it's a good idea to add a unique constraint on one side of
the one-to-one relationship:
class Face {
static hasOne = [nose:Nose]
static constraints = {
nose unique: true
}
}
class Nose {
Face face
}
Yes you can, but it behave differently
class Author {
static hasMany = [books: Book]
String name
}
class Book {
String title
}
In this case if you delete Author the books still existing and are independent.
class Author {
static hasMany = [books: Book]
String name
}
class Book {
String title
static belongsTo = [author: Author]
}
In this other case if you delete the Author it will delete all the books pf that author in cascade.
Many-to-one/one-to-one: saves and deletes cascade from the owner to the dependant (the class with the belongsTo).
One-to-many: saves always cascade from the one side to the many side, but if the many side has belongsTo, then deletes also cascade in that direction.
Many-to-many: only saves cascade from the "owner" to the "dependant", not deletes.
http://grails.org/doc/2.3.x/ref/Domain%20Classes/belongsTo.html
yes very easy like a class defintion but only specify hasMany but no need for hasOne
class Student {
String name
User userProfile
static hasMany =[files:File]
}
class User {
String uname
Student student
}
class File {
String path
Student student // specify the belongs to like this no belong to
}
Done!!
Related
What is the difference between
static belongsTo [author:Author]
and
static belongsTo = Author
Lets consider two domain class.
Class Author{
String name
}
Class Books{
String name
static belongsTo = Author
}
When static belongsTo = Author is kept in Books domain, it have no effects on db. However, static belongsTo = [author : Author] creates a backreference to Author class and also there is author_id column in db. So, what actually using static belongsTo = Author alone does.
This is expalined in grails docs(http://grails.github.io/grails-doc/latest/ref/Domain%20Classes/belongsTo.html).
Also, what is the difference between using following two :
Class Books{
String name
static belongsTo = [author : Author]
}
Class Books{
String name
Author author
}
static belongsTo = [author : Author] is used only for cascading purposes, Is it true or does it have different use cases.
Can anyone explain these in detail without relating it to hasOne or hasMany. Thanks in advance.
Using belongsTo without a back-reference is necessary for many-to-many associations. For example, lets assume you have the following domain classes:
class Book {
String name
Author author
static hasMany = [categories: Category]
}
class Category {
String name
static hasMany = [books: Book]
}
If you try to use them as-is you'd get an exception like this:
No owner defined between domain classes [class Book] and [class Category] in a many-to-many relationship. Example: static belongsTo = Category
The solution is to make one of the domain classes the owner of the many-to-many association. Using my example, I think it makes more sense to make the Category the owner. However, a back-reference would not work because there could be multiple Categorys. So, this is where a belongsTo without a back-reference comes in:
class Book {
String name
Author author
static hasMany = [categories: Category]
static belongsTo = Category
}
If you use static belongsTo = [author: Author] then a property named author of type Author is added to the class as a back reference. With static belongsTo = Author that does not happen.
I am using Grails 3.0.6 and am struggling with a complicated and highly interconnected domain model. I have classes with multiple many-to-many associations to other classes and I am left with no choice but to have multiple belongsTo associations on at least one class. I am unable to figure out the syntax to represent this.
My domain model was quite complicated, but I was able to reduce my problem to this simplified example:
class Graph {
static hasMany = [vertices: Vertex]
}
class OtherClass {
static hasMany = [vertices: Vertex]
}
class Vertex {
static hasMany = [graph: Graph, other: OtherClass]
}
In this simplified example, I could get around the problem by declaring the ownership between the domain classes on Graph and OtherClass... In my complicated domain model, I don't have this choice because there are too many classes with multiple many-to-many associations.
I have tried this:
class Vertex {
static hasMany = [graphs: Graph, others: OtherClass]
static belongsTo = Graph, OtherClass
}
but I get an NPE.
I have tried this:
class Vertex {
static hasMany = [graphs: Graph, others: OtherClass]
static belongsTo = [graphs: Graph, others: OtherClass]
}
but I still get "GrailsDomainException: No owner defined between domain classes [Graph] and [Vertex]"
Is there something I could do with mappedBy to correctly represent this?
In many of my many-to-many associations, cascading saves are not actually wanted (although they won't hurt), so I don't need belongsTo (or an "owner") for that purpose. This makes me wonder if associations on the domain classes are really how I should be modeling these relationships. Is there something else I could be doing?
per Burt Beckwith's comment, I created an additional domain class to represent the join table. Now, one many-to-many association is broken down into two one-to-many associations and the problem does not arise.
Example:
class Graph {
static hasMany = [graphVertexRelations: GraphVertexRelation]
}
class OtherClass {
static hasMany = [vertices: Vertex]
}
class Vertex {
static hasMany = [graphVertexRelations: GraphVertexRelation, others: OtherClass]
static belongsTo = OtherClass
}
class GraphVertexRelation {
static belongsTo = [graph: Graph, vertex: Vertex]
static GraphVertexRelation create(Graph graph, Vertex vertex, boolean flush = false) {
new GraphVertexRelation(graph: graph, vertex: vertex).save(flush: flush, insert: true)
}
}
The exception that you're seeing "GrailsDomainException: No owner defined between domain classes [Graph] and [Vertex]" means that the ORM can't figure out what the base class is, and there is a cyclical relationship between Graph and Vertex.
If you want to maintain the relationship to see what Graph a Vertex is in, you can do a backwards lookup using a criteria.
class Graph {
static hasMany = [vertices: Vertex]
}
class OtherClass {
static hasMany = [vertices: Vertex]
}
class Vertex {
static transients = ['graphs']
static hasMany = [other: OtherClass]
List<Graph> getGraphs() {
// Backwards link, using the graph table
Graph.withCriteria() {
vertices {
inList("id", [this.id.toLong()])
}
}
}
}
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).
My project requires me to maintain the insertion and retrieval order in a many-many relationship. By default, groovy saves the elements as a Set in many-many relationship. I want to do it in a List. I am not sure how to update the relationship to use List instead of Set. Any help would be appreciated.
class Course{
static belongsTo = Teacher
static hasMany = [teacher:Teacher]
static mapping = {
teacher joinTable : [name: TeacherCourse]
}
}
class Teacher{
static hasMany = [course:Course]
static mapping = {
course joinTable : [name: TeacherCourse]
}
}
save() call on either Teacher or Course also inserts a new row in TeacherCourse table. It works with no issues. In Database there the tables are:-
Teacher (PK: Id)
Course (PK: Id)
TeacherCourse(PK: [Teacher_id,Course_id])
Is there a way I can maintain the order of insertion and retrieval in many-many relationship?
Thank you..
Edit
In controller save()
def courseInstance = new Course()
List <Teacher> teacherList= []
teacherList.add(Teacher.findById(65))
teacherList.add(Teacher.findById(36))
courseInstance.courseUnits = teacherList
courseInstance.save(flush:true)
Try this:
class Course {
List teachers
static belongsTo = Teacher
static hasMany = [teachers:Teacher]
static mapping = {
teachers joinTable : [name: TeacherCourse]
}
}
class Teacher {
List courses
static hasMany = [courses:Course]
static mapping = {
courses joinTable : [name: TeacherCourse]
}
}
Reference
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