Grails Hibernate criteria - Checking if a relation exists

Question

I'm attempting to perform a filter to find if a certain object has a relationship with another. I had an object Component :

class Component { 
    long id
    String name
}

And a Component is used by a Page.

class Page {
    long id
    static hasMany = [components : Component]
}

How would I go about building a hibernate criteria to check if my component has a relationship with any page? One component can be used by many pages. So far, all I can think of is something like:

Component.createCriteria().list {
    inList("id", Page.list().components);
}

But that isn't going to scale very nicely at all. So I was wondering if there was a simpler way of saying "If my object is used by this object"?

Answer 1

Page.createCriteria().count {
    components {
        eq("id", yourComponent.id)
    }
}

This query gives you the count of all the Pages which have yourComponent. And if this is 0 then there yourComponent is not associated to any Page.

EDIT____________________________________________________________

No, because Component have no relation with Page .

But if you want this then my suggestion is formula column . In this relationship you have three tables component , page and there relationship table page_components .

eg.,

class Component { 
    long id
    String name
    Boolean isRelatedWithAnyPage

    static mapping = {
        isRelatedWithAnyPage formula: "(select (count(*)>0) from page_components pc where pc.component_id = id)"
    }
}

Note:- I haven't tried this, may be you need to change the sql query in formula.

Answer 2

另一种方法是创建一个PageComponent域，以匹配grails自动创建的基础联接表，并执行PageComponent.countByComponentId(componentId)

Answer 3

You can use exists with a subquery.

def subQuery = Page.where({
   components {
      eqProperty("id", "this.id") //this: is the root alias which refers to the Component
   }
   setAlias("page")
})


Component.createCriteria().list {
  exists(subQuery.id())
}

See how to execute exists subqueries with grails criteria