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POGet collection count instead of the collection in JPA Criteria API?
 

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  1. POGet collection count instead of the collection in JPA Criteria API?
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    <p>Lets say I have an entity:</p> <pre><code>@Entity public class Person { @Id @GeneratedValue private Long id; @ManyToMany(fetch = FetchType.LAZY) private List&lt;Role&gt; roles; @ManyToMany(fetch = FetchType.LAZY) private List&lt;Permission&gt; permissions; // etc // other fields here } </code></pre> <p>I want a to build a query using the Criteria API that filters these users and shows a list of people and among other info from the entity - how many roles does a person have.</p> <pre><code>CriteriaBuilder builder = em.getCriteriaBuilder(); CriteriaQuery&lt;Person&gt; query = builder.createQuery(Person.class); Root&lt;Person&gt; personRoot = query.from(Person.class); // predicates here </code></pre> <p>However, this limits me to returning only a list of <code>Person</code> entities. I can always add a <code>@Transient</code> field to the entity, but this seems ugly since I might have many different queries and might end up with many such fields.</p> <p>On the other hand - I cant use <code>HQL</code> and write the query since I want complex filtering and I would have to deal with appending and removing things from the <code>HQL</code> query.</p> <p>My question, besides the one in the title of this post is this: how do I query the database using the Criteria API and return a non-entity (in case I want to filter the Person table but return only the number of roles, permissions, etc) and how do I do it for something very close to the actual entity (like the example with the role counter instead of the roles collection)?</p> <p><strong>UPDATE</strong> Using Hibernate's projections I came up with this. But still don't know that to write in <code>TODO</code>. <code>Projections.count</code> doesn't work since it excpects some kind of grouping, and I don't seem to be able to find any examples in the <code>Hibernate</code> documentation.</p> <pre><code>Criteria cr = session.createCriteria(Person.class); if (id != null) { cr.add(Restrictions.eq("id", id)); } ProjectionList projectionList = Projections.projectionList(); projectionList.add(Projections.property("id"), "id"); projectionList.add(TODO, "rolesCount"); </code></pre>
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    1. COAre you looking for something like [this answer](http://stackoverflow.com/a/14676881/785663)?
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    2. COOr something like the following: http://stackoverflow.com/questions/19707529/jpa-select-count-distinct/19708805#19708805 ?
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    3. COThese two answers return a count/sum, which is only a part of what I need. I can always have multiple querries - for example one filtering the entities and one counting the roles and then mapping the rwo results, but I'm looking for a practical way to do these in one Criteria query.
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