Thursday, June 21, 2012

Eager fetching grails and Hibernate

This is taken from grails docs  
 Basically for single associations use fetch: 'join' (using lazy:false means that there will be 2 sql statements per association.). However for one to many use lazy:false

Configuring Eager Fetching

An alternative approach that avoids the N+1 queries is to use eager fetching, which can be specified as follows:
class Airport {
    String name
    static hasMany = [flights: Flight]
    static mapping = {
        flights lazy: false
In this case the flights association will be loaded at the same time as its Airport instance, although a second query will be executed to fetch the collection. You can also use fetch: 'join' instead of lazy: false , in which case GORM will only execute a single query to get the airports and their flights. This works well for single-ended associations, but you need to be careful with one-to-manys. Queries will work as you'd expect right up to the moment you add a limit to the number of results you want. At that point, you will likely end up with fewer results than you were expecting. The reason for this is quite technical but ultimately the problem arises from GORM using a left outer join.
So, the recommendation is currently to use fetch: 'join' for single-ended associations and lazy: false for one-to-manys.
Be careful how and where you use eager loading because you could load your entire database into memory with too many eager associations. You can find more information on the mapping options in the section on the ORM DSL.

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