Hibernate Criteria连接到包含外键的表

Question

I have two tables:

Client (clientId, firstName, lastName, gender)

Event (clientId, eventId)

I need to represent a query similar to the following using Criteria:

SELECT c.clientId, c.firstName, c.lastName, c.gender, MAX(eventId)
FROM Client c JOIN Event e ON c.clientId = e.clientId
GROUP BY c.clientId, c.firstName, c.lastName, c.gender

I have tried this:

final Criteria criteria = session.createCriteria(Client.class);
criteria.setFetchMode("Event", FetchMode.JOIN);
criteria.setProjection(Projections.projectionList().add(Projections.groupProperty("clientId")).add(Projections.max("eventId")));

but it throws an exception on the last line with the message:

HibernateQueryException: could not resolve property: eventId of: Client

How can I specify the join between the Client table which itself contains no column related to the Event table but the clientId column on the Event table is a foreign key back into the Client table?

As you can see, it's really driven off the Client table and that I only need to select the maximum eventId from the Event table. Also, as I mentioned, I am trying to make a change to an existing Criteria query which is based on the Client class. It is used to retrieve all the columns for all active clients. I just need to add one extra column to the query results - the maximum eventId .

Answer 1

Use alias

Criteria criteria = session.createCriteria(Event.class, "et").
createAlias("et.Client", "ct").
setProjection(Projections.projectionList().         
add(Projections.groupProperty("et.clientId")).
add(Projections.max("et.eventId")));

For more details on criteria, refer Criteria Queries

Answer 2

That is obvious. Because Client class does not have eventId property, and your criteria is defined for Client class.

When trying to use a property of B class inside a Criteria for A, you have to use Aliases.

All you have to do is to modify your code like this:

final Criteria criteria = session.createCriteria(Event.class, "event");
criteria.createAlias("event.client", "client");
criteria.setProjection(Projections.projectionList().add(Projections.groupProperty("clientId")).add(Projections.max("eventId")));

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UPDATED (based on your comment)

As your query needs Event class, you have to have a Criteria for this class. So you have to something like this:

 final Criteria criteria = session.createCriteria(Event.class, "event"); criteria.createAlias("event.client", "client"); //The criteria below, is returning clientId DetachedCriteria eventCr = DetachedCriteria.forClass(Event.class, "event"); eventCr.setProjection(Projections.projectionList().add(Projections.groupProperty("clientId")).add(Projections.max("eventId"))); //Now using subqueries you can achieve your goal criteria.add(Subqueries.propertyIn("clientId", eventCr)); 

I don't know for sure what you're looking for, but I hope I have given you some good hints. You might want to try Subqueries.propertyEq instead if your query must return a single id.