NHibernate How do I query against an IList

问题:

I am trying to query against an IList<string> property on one of my domain classes using NHibernate. Here is a simple example to demonstrate:

public class Demo
{
    public Demo()
    {
        this.Tags = new List<string>();
    }
    public virtual int Id { get; set; }
    public virtual string Name { get; set; }
    public virtual IList<string> Tags { get; set; }
}

Mapped like this:

<class name="Demo">
<id name="Id" />
<property name="Name" />
<bag name="Tags">
  <key column="DemoId"/>
  <element column="Tag" type="String" />
</bag>

And I am able to save and retrieve just fine. Now to query for instances of my domain class where the Tags property contains a specified value:

var demos = this.session.CreateCriteria<Demo>()
            .CreateAlias("Tags", "t")
            .Add(Restrictions.Eq("t", "a"))
            .List<Demo>();

Results in the error: collection was not an association: Demo.Tags

var demos = (from d in this.session.Linq<Demo>()
                     where d.Tags.Contains("a")
                     select d).ToList();

Results in the error: Objct reference not set to an instance of an object.

var demos = this.session.CreateQuery("from Demo d where :t in elements(d.Tags)")
            .SetParameter("t", "a")
            .List<Demo>();

Works fine, but as my real domain class has many many properties, and I am building a complicated dynamic query, doing ugly string manipulation is not my first choice. I'd much rather use ICriteria or Linq. I have a user interface where many different possible search criteria can be entered. The code that builds up the ICriteria right now is dozens of lines long. I'd really hate to turn that into HQL string manipulation.

回答1:

So because of limitations of the Criteria API, I decided to bend my domain classes to fit.

I created an entity class for the Tag. I couldn't even create it as a value object. It had to have its own id.

I feel dirty now. But being able to construct a dynamic query without resorting to string manipulation was more important to me than staying true to the domain.

回答2:

As documented here:

17.1.4.1. Alias and property references

we can use:

...
A collection key             {[aliasname].key}      ORGID as {coll.key}
The id of an collection      {[aliasname].id}       EMPID as {coll.id}
The element of an collection {[aliasname].element}  XID as {coll.element}
...

there is a small bug in doc... instead of ".element" we have to use ".elements"

var demos = this.session.CreateCriteria<Demo>()
        .CreateAlias("Tags", "t")

        // instead of this
        // .Add(Restrictions.Eq("t", "a"))

        // we can use the .elements keyword
        .Add(Restrictions.Eq("t.elements", "a"))

        .List<Demo>();

回答3:

You need to use SubCriterias not alias. This should work:

var demos = this.session.CreateCriteria<Demo>()
            .CreateCriteria("Tags")
            .Add(Restrictions.Eq("Tag", "a"))
            .List<Demo>();

回答4:

HQL:

from Demo d where :val in elements(d.Tags)

回答5:

Switching to a class over a string is one compromise. Using HQL instead of ICriteria is another. There is a third compromise however... use custom SQL. Try this out.

var demos = Session.CreateCriteria<Demo>()
    .Add(Expression.Sql(
        "EXISTS (SELECT 1 FROM [Tags] custom_sql_t WHERE custom_sql_t.[DemoId] = {alias}.[Id] AND custom_sql_t.[Tag] = ?)",
        "a",
        NHibernateUtil.String))
    .List<Demo>();

This results in the follwing SQL being generated by NHibernate 2.1.2.4000...

exec sp_executesql N'SELECT this_.Id as Id2_0_, this_.Version as Version2_0_, this_.Name as Name2_0_ FROM Demo this_ WHERE EXISTS (SELECT 1 FROM [Tags] custom_sql_t WHERE custom_sql_t.[DemoId] = this_.[Id] AND custom_sql_t.[Tag] = @p0)',N'@p0 nvarchar(1)',@p0=N'a'

See this post for another example...

NHibernate - Querying from a collection of Value Types (non-Entity) to solve Select N+1

回答6:

This is possible by creating a separate criteria:

ICriteria demoCriteria = session.CreateCriteria<Demo>();
...
demoCriteria.Add(Restrictions...);
...
ICriteria tagCriteria = demoCriteria.CreateCriteria("Tags");
tagCriteria.Add(Restrictions.In("elements", new {"Tag1", "Tag2", ...}));

return demoCriteria.List<Demo>();