Grails

hasOne results in eager fetching of association

Details

  • Type: Bug Bug
  • Status: Closed Closed
  • Priority: Major Major
  • Resolution: Won't Fix
  • Affects Version/s: 1.3.3
  • Fix Version/s: None
  • Component/s: None
  • Labels:
    None

Description

If I have the following domain model:

class Author {
    String name
    
    static hasOne = [ location: Location ]
}

class Location {
    String city
}

then when I execute this code:

Author.list().each { a ->
    println a.location.city
}

I find that the location instances are all loaded up front with separate SELECTs. In other words, they're loaded eagerly. Explicitly specifying the relationship as lazy: true has no effect.

Perhaps this behaviour is expected, but it seems strange to me.

Issue Links

duplicates

Bug - A problem which impairs or prevents the functions of the product. GRAILS-5077 hasOne mapping is by default eager and cannot be changed to lazy

  • Blocker - Blocks development and/or testing work, production could not run.
  • In Progress - This issue is being actively worked on at the moment by the assignee.
is related to

Bug - A problem which impairs or prevents the functions of the product. GRAILS-6538 Bidirectional 'one-to-one' mapping results in extra unnecessary query on relationship

  • Major - Major loss of function.
  • Open - The issue is open and ready for the assignee to start work on it.

Activity

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Peter Ledbrook added a comment -

Attaching an example project. Run the Grails console and then execute

new BootStrap().init()

followed by

Author.list().each { a ->
    println a.name
}

Even though the above code doesn't use the location at all, you'll see in the console output that the locations for each author are loaded up front.

Show
Peter Ledbrook added a comment - Attaching an example project. Run the Grails console and then execute new BootStrap().init() followed by Author.list().each { a -> println a.name } Even though the above code doesn't use the location at all, you'll see in the console output that the locations for each author are loaded up front.
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Jeff Brown added a comment -

I have stepped into this a bit and I see that the fetch mode on the corresponding DefaultGrailsDomainClassProperty object is GrailsDomainClassProperty.FETCH_LAZY (0). I am still trying to figure out why the relationship is being eagerly fetched.

Show
Jeff Brown added a comment - I have stepped into this a bit and I see that the fetch mode on the corresponding DefaultGrailsDomainClassProperty object is GrailsDomainClassProperty.FETCH_LAZY (0). I am still trying to figure out why the relationship is being eagerly fetched.
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Jeff Brown added a comment -

I have found that even if I use the Hibernate API directly to retrieve an Author, its Location is eagerly fetched. That tells me that the eager fetching is not being triggered by GORM query code.

Show
Jeff Brown added a comment - I have found that even if I use the Hibernate API directly to retrieve an Author, its Location is eagerly fetched. That tells me that the eager fetching is not being triggered by GORM query code.
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Peter Ledbrook added a comment -

To add to the confusion, GRAILS-6538 suggests that hasOne results in no extra query.

Show
Peter Ledbrook added a comment - To add to the confusion, GRAILS-6538 suggests that hasOne results in no extra query.
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Peter Ledbrook added a comment -

OK, ignore that. GRAILS-6538 is talking about an extra query to fetch the Author from the associated location. Different (albeit related) issue.

Show
Peter Ledbrook added a comment - OK, ignore that. GRAILS-6538 is talking about an extra query to fetch the Author from the associated location. Different (albeit related) issue.
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Peter Ledbrook added a comment -

http://community.jboss.org/wiki/AShortPrimerOnFetchingStrategies

(The exception is a one-to-one association with a shared primary key: a proxy for the target of an association can only be used if the target is required. If a Person has a one-to-one association to a Desk entity, the primary key value of the Desk must be the same as the primary key value of the Person. The Desk can only be proxied if the Person primary key column has a foreign key constraint referencing the primary key column of Desk, making it non-optional and required; a Person needs a Desk. If this foreign key constraint is not present, or if the foreign key constraint is not mapped in Hibernate with constrained="true" in XML or optional="false" in annotations, no proxy for a Desk can be used, and Hibernate has to hit the database, either with a join or a secondary immediate query, to find out if the association is really null or not. You can use bytecode injected interception to get lazy loading in that case. Read this page for an alternative explanation of the same issue.)

So there are some one-to-one mappings in which the association will always be eagerly fetched. Perhaps the issue is that the foreign key constraint is not mapped correctly? Or the 'constrained' is not true?

Show
Peter Ledbrook added a comment - http://community.jboss.org/wiki/AShortPrimerOnFetchingStrategies (The exception is a one-to-one association with a shared primary key: a proxy for the target of an association can only be used if the target is required. If a Person has a one-to-one association to a Desk entity, the primary key value of the Desk must be the same as the primary key value of the Person. The Desk can only be proxied if the Person primary key column has a foreign key constraint referencing the primary key column of Desk, making it non-optional and required; a Person needs a Desk. If this foreign key constraint is not present, or if the foreign key constraint is not mapped in Hibernate with constrained="true" in XML or optional="false" in annotations, no proxy for a Desk can be used, and Hibernate has to hit the database, either with a join or a secondary immediate query, to find out if the association is really null or not. You can use bytecode injected interception to get lazy loading in that case. Read this page for an alternative explanation of the same issue.) So there are some one-to-one mappings in which the association will always be eagerly fetched. Perhaps the issue is that the foreign key constraint is not mapped correctly? Or the 'constrained' is not true ?
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Dana Berke added a comment -

This appears to be a duplicate of GRAILS-5077.

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Dana Berke added a comment - This appears to be a duplicate of GRAILS-5077 .
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Jeff Brown added a comment -

Duplicates GRAILS-5077 which is marked as Won't Fix.

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Jeff Brown added a comment - Duplicates GRAILS-5077 which is marked as Won't Fix.

People

  • Assignee:
    Jeff Brown
    Reporter:
    Peter Ledbrook
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Dates

  • Created:
    Updated:
    Resolved: