Fix exists() cross-join caused by duplicate CriteriaQuery root

[jamesfredley]

Summary

Fixes #14334

GormEntity.exists() was performing a cartesian product (cross-join) against the entire table due to criteriaQuery.from() being called twice in AbstractHibernateGormStaticApi.exists().

Reproducer: https://github.com/scottlollman/grails-data-mapping-issue-2071

Root Cause

AbstractHibernateGormStaticApi.exists() created two query roots:

Root queryRoot = criteriaQuery.from(persistentEntity.javaClass)          // root 1
// ...
criteriaQuery.select(criteriaBuilder.count(criteriaQuery.from(...)))     // root 2 (BUG)

Each call to criteriaQuery.from() adds a new root to the query. Two roots on the same table produce a cross-join:

-- Before (cross-join scans entire table)
select count(generatedAlias0)
from SomeDomain as generatedAlias1, SomeDomain as generatedAlias0
where generatedAlias1.id=1L

-- After (single root, simple indexed lookup)
select count(generatedAlias0)
from SomeDomain as generatedAlias0
where generatedAlias0.id=1L

While the boolean result was technically correct (non-zero = true), the query performed a full table scan for every exists() call.

Fix

One-line change - reuse the existing queryRoot variable instead of calling criteriaQuery.from() a second time:

criteriaQuery.select(criteriaBuilder.count(queryRoot))

Tests

Unit Tests (ExistsCrossJoinSpec) - 4 tests

Integration tests using a standalone HibernateDatastore with H2:

• exists returns true for existing entity
• exists returns false for non-existent id
• exists does not produce a cross-join (SQL captured via Hibernate StatementInspector, verified no cross-join or comma-join pattern)
• exists with multiple rows returns correct result

Functional Tests (ExistsSpec) - 3 tests

Full Grails integration tests in grails-test-examples/gorm using the existing Product domain class:

• exists returns true for persisted entity
• exists returns false for non-existent id
• exists returns correct result with multiple rows in table

Existing ReadOperationSpec tests continue to pass.

[Copilot]

Pull request overview

Fixes a Hibernate CriteriaQuery inefficiency in GormEntity.exists() where calling criteriaQuery.from() twice created duplicate roots and caused a cartesian product (cross-join), leading to full table scans.

Changes:

• Reuse the existing CriteriaQuery root in AbstractHibernateGormStaticApi.exists() to avoid introducing a second root (and cross-join).
• Add a core test (ExistsCrossJoinSpec) that captures SQL via StatementInspector to assert no cross-join/comma-join is generated for exists().
• Add an integration test example (ExistsSpec) in grails-test-examples/gorm validating exists() behavior in a full Grails app context.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
grails-data-hibernate5/core/src/main/groovy/org/grails/orm/hibernate/AbstractHibernateGormStaticApi.groovy	Fixes the duplicate-root CriteriaQuery construction by counting the existing queryRoot.
grails-data-hibernate5/core/src/test/groovy/org/grails/orm/hibernate/ExistsCrossJoinSpec.groovy	Adds regression coverage to ensure exists() does not generate cross-joins in SQL.
grails-test-examples/gorm/src/integration-test/groovy/gorm/ExistsSpec.groovy	Adds functional/integration validation of exists() in the sample app.

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[jdaugherty]

I'm assuming the test failure is due to test flakyness