What is the difference between JoinQueryOver and JoinAlias?
Functionally they do the same thing, create a join to another entity. The only difference is what they return. JoinQueryOver returns a new QueryOver with the current entity being the entity joined, while JoinAlias returns the original QueryOver that has the current entity as the original root entity.
Whichever one you use is a matter of personal taste: (from http://nhibernate.info/doc/nh/en/index.html#queryqueryover)
IQueryOver<Cat,Kitten> catQuery =
session.QueryOver<Cat>()
.JoinQueryOver<Kitten>(c => c.Kittens)
.Where(k => k.Name == "Tiddles");
and
Cat catAlias = null;
Kitten kittenAlias = null;
IQueryOver<Cat,Cat> catQuery =
session.QueryOver<Cat>(() => catAlias)
.JoinAlias(() => catAlias.Kittens, () => kittenAlias)
.Where(() => kittenAlias.Name == "Tiddles");
Are functionally the same. Note how the kittenAlias is expressly referenced in the second query.
QueryOver Series - Part 2: Basics and Joining by Andrew Whitaker gives a very good explanation:
Summary:
IQueryOver
is a generic type with two type parametersTRoot
andTSubType
.Select
operates onTRoot
while other QueryOver methods operate onTSubType
.TRoot
stays the same as you’re building a query, butTSubType
changes when you join usingJoinQueryOver
JoinQueryOver
andJoinAlias
add joins to your query.JoinAlias
doesn’t changeTSubType
, butJoinQueryOver
does.- You can use aliases when building a query to refer to properties that don’t belong to
TRoot
orTSubType