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NHibernate Pitfalls: QueryOver and Deep Expressions

This is part of a series of posts about NHibernate Pitfalls. See the entire collection here.

When using the QueryOver API, unlike LINQ, when applying restrictions, you only have access to direct properties of the target entity type. If you need to access deeper nested properties – those from associated entities – you have to explicitly join. This does not apply to the property representing the foreign key, however. Let’s see an example:

   1: //this will not work
   2: session.QueryOver<Order>().Where(x => x.Customer.Name == "Some Name").List();
   3:  
   4: //this will work
   5: session.QueryOver<Order>().JoinQueryOver(x => x.Customer).Where(x => x.Name == "Some Name").List();
   6:  
   7: //this will also work because Id is the foreign key
   8: session.QueryOver<Order>().Where(x => x.Customer.Id == CustomerId).List();

In general, the QueryOver API is more complex to use than the LINQ one, the advantage is that it allows better composition of dynamic queries.

Comments

hazzik said:

>the advantage is that it allows better composition of dynamic queries.

What you mean? I dont see any advantages on this.

# August 23, 2012 8:46 AM

Ricardo Peres said:

Hazzik:

All/most of its methods have overloads for ICriterion/IProjection parameters, where you can build actual instances passing literals, also, you can specify join types explicitly (unlike LINQ) which are much better for dynamic construction of queries, don't you think? I know about dynamic LINQ and have talked about it, but it requires external libraries.

# August 23, 2012 1:51 PM