NHibernate LinqToHqlGenerator for SQL Server 2008 full text index 'Containing' keyword - sql-server-2008

Question

I think I'm missing something fundamental when implementing a LinqToHql generator class.
I've successfully registered the SQL Server 2008 contains query using a custom dialect with this registration:
RegisterFunction("contains", new StandardSQLFunction("contains", null));
I have only one class with a full text index to be queried:
public class SearchName
{
public virtual Guid Id {get; set;}
public virtual string Name {get; set;} // this is the search field
}
The contains function works properly in HQL:
var names = Session.CreateQuery("from SearchName where contains(Name,:keywords)")
.SetString("keywords", "john")
.List();
and the generated SQL is perfect:
select searchname0_.Id as Id4_,
searchname0_.Name as Name4_
from Search_Name searchname0_
where contains(searchname0_.Name, 'john' /* #p0 */)
The next challenge was to implement the Linq to HQL generator:
public class MyLinqtoHqlGeneratorsRegistry :
DefaultLinqToHqlGeneratorsRegistry
{
public MyLinqtoHqlGeneratorsRegistry()
{
this.Merge(new ContainsGenerator());
}
}
public class ContainsGenerator : BaseHqlGeneratorForMethod
{
public ContainsGenerator()
{
SupportedMethods = new[] {
ReflectionHelper.GetMethodDefinition<SearchName>(d => d.Name.Contains(String.Empty))
};
}
public override HqlTreeNode BuildHql(MethodInfo method,
System.Linq.Expressions.Expression targetObject,
ReadOnlyCollection<System.Linq.Expressions.Expression> arguments,
HqlTreeBuilder treeBuilder, IHqlExpressionVisitor visitor)
{
return treeBuilder.MethodCall("contains",
visitor.Visit(targetObject).AsExpression(),
visitor.Visit(arguments[0]).AsExpression()
);
}
}
}
Calling the method like this:
var namesLinq = Session.Query<SearchName>().Where(x=> x.Name.Contains("john")).ToList();
Unfortunately, this doesn't seem to override the built-in Contains method, and the generated SQL is wrong:
select searchname0_.Id as Id4_,
searchname0_.Name as Name4_
from Search_Name searchname0_
where searchname0_.Name like ('%' + 'john' /* #p0 */ + '%')
Is it not possible to override the default Contains method, or have I just made a silly mistake?
PS - I'm using NHibernate 3.3.1.4000

Answer 1

OK, I've finally figured it out!
First, I managed to delete the registration code from my configuration:
...
.ExposeConfiguration(cfg =>
{
cfg.LinqToHqlGeneratorsRegistry<MyLinqtoHqlGeneratorsRegistry>();
...
}
Second, don't try to override the existing Linq behaviors. I moved my Contains extension method to the full-text class.
Third, build the Hql tree correctly.
For others trying to implement a SQL 2008 Free-text contains search, here's the complete implementation:
public static class DialectExtensions
{
public static bool Contains(this SearchName sn, string searchString)
{
// this is just a placeholder for the method info.
// It does not otherwise matter.
return false;
}
}
public class MyLinqtoHqlGeneratorsRegistry : DefaultLinqToHqlGeneratorsRegistry
{
public MyLinqtoHqlGeneratorsRegistry()
: base()
{
RegisterGenerator(ReflectionHelper.GetMethod(() =>
DialectExtensions.Contains(null, null)),
new ContainsGenerator());
}
}
public class ContainsGenerator : BaseHqlGeneratorForMethod
{
string fullTextFieldName = "Name";
public ContainsGenerator()
: base()
{
SupportedMethods = new[] {
ReflectionHelper.GetMethodDefinition(() =>
DialectExtensions.Contains(null, null))
};
}
public override HqlTreeNode BuildHql(MethodInfo method,
System.Linq.Expressions.Expression targetObject,
ReadOnlyCollection<System.Linq.Expressions.Expression> arguments,
HqlTreeBuilder treeBuilder, IHqlExpressionVisitor visitor)
{
// cannot figure out how to interrogate the model class to get an
// arbitrary field name...
// perhaps the RegisterGenerator() call above could be used to pass a
// property name to the ContainsGenerator constructor?
// in our case, we only have one full text searchable class, and its
// full-text searchable field is "Name"
HqlExpression[] args = new HqlExpression[2] {
treeBuilder.Ident(fullTextFieldName).AsExpression(),
visitor.Visit(arguments[1]).AsExpression()
};
return treeBuilder.BooleanMethodCall("contains", args);
}
}
For the above to work, you must have declared and used your custom dialect:
public class CustomMsSql2008Dialect : NHibernate.Dialect.MsSql2008Dialect
{
public CustomMsSql2008Dialect()
{
RegisterFunction(
"contains",
new StandardSQLFunction("contains", null)
);
}
}
Then you can use your new contains search this way:
var namesLinq = Session.Query<SearchName>().Where(x => x.Contains("john")).ToList();
... and the resulting SQL is perfect! (at least if you only have one table you're performing full-text searches on)
EDIT: UPDATED IMPLEMENTATION TO SUPPORT MORE THAN ONE FULLTEXT 'Contains' SEARCH PER QUERY.
Here's the revised version:
public static class DialectExtensions
{
public static bool FullTextContains(this string source, string pattern)
{
return false;
}
}
public class MyLinqtoHqlGeneratorsRegistry : DefaultLinqToHqlGeneratorsRegistry
{
public MyLinqtoHqlGeneratorsRegistry()
: base()
{
RegisterGenerator(ReflectionHelper.GetMethod(() => DialectExtensions.FullTextContains(null, null)),
new FullTextContainsGenerator());
}
}
public class FullTextContainsGenerator : BaseHqlGeneratorForMethod
{
public FullTextContainsGenerator()
{
SupportedMethods = new[] { ReflectionHelper.GetMethod(() => DialectExtensions.FullTextContains(null, null)) };
}
public override HqlTreeNode BuildHql(MethodInfo method,
System.Linq.Expressions.Expression targetObject,
ReadOnlyCollection<System.Linq.Expressions.Expression> arguments,
HqlTreeBuilder treeBuilder, IHqlExpressionVisitor visitor)
{
HqlExpression[] args = new HqlExpression[2] {
visitor.Visit(arguments[0]).AsExpression(),
visitor.Visit(arguments[1]).AsExpression()
};
return treeBuilder.BooleanMethodCall("contains", args);
}
}
To use the revised version, the syntax is slightly different:
var namesLinq = Session.Query<SearchName>().Where(x => x.Name.FullTextContains("john")).ToList();