Sometimes you will need a more sophisticated way to translate method calls to HQL. In this recipe, we will show you how to implement a custom LINQ to HQL generator that is able to do this.
- Create a new class library project named
CustomLinqGenearatorExample. - Install the
NHibernateandlog4netpackages using the NuGet Package Manager Console by executing the following command:Install-Package NHibernate Install-Package log4net
- Create the
DateTimeFunctionsGeneratorclass using the following code:public class DateTimeFunctionsGenerator : BaseHqlGeneratorForMethod { public DateTimeFunctionsGenerator() { SupportedMethods = new[] { ReflectionHelper.GetMethod<DateTime>( d => d.AddDays(0)), ReflectionHelper.GetMethod<DateTimeOffset>( d => d.AddDays(0)), ReflectionHelper.GetMethod<DateTime>( d => d.AddHours(0)), ReflectionHelper.GetMethod<DateTimeOffset>( d => d.AddHours(0)), ReflectionHelper.GetMethod<DateTime>( d => d.AddMinutes(0)), ReflectionHelper.GetMethod<DateTimeOffset>( d => d.AddMinutes(0)), ReflectionHelper.GetMethod<DateTime>( d => d.AddSeconds(0)), ReflectionHelper.GetMethod<DateTimeOffset>( d => d.AddSeconds(0)), ReflectionHelper.GetMethod<DateTime>( d => d.AddMilliseconds(0)), ReflectionHelper.GetMethod<DateTimeOffset>( d => d.AddMilliseconds(0)), }; } public override HqlTreeNode BuildHql( MethodInfo method, Expression targetObject, ReadOnlyCollection<Expression> arguments, HqlTreeBuilder treeBuilder, IHqlExpressionVisitor visitor) { return treeBuilder.MethodCall( method.Name, visitor.Visit(targetObject).AsExpression(), visitor.Visit(arguments[0]).AsExpression()); } } - Create the
CustomLinqtoHqlGeneratorsRegistryclass using the following code:public class CustomLinqtoHqlGeneratorsRegistry : DefaultLinqToHqlGeneratorsRegistry { public CustomLinqtoHqlGeneratorsRegistry() { this.Merge(new DateTimeFunctionsGenerator()); } } - Register the
CustomLinqtoHqlGeneratorsRegistryinto a configuration using the following code:cfg.SetProperty( Environment.LinqToHqlGeneratorsRegistry, typeof (CustomLinqtoHqlGeneratorsRegistry). AssemblyQualifiedName)
In the constructor of DateTimeFunctionsGenerator, we define the methods that are supported by this HQL generator. In this case, we want to be able to handle AddDays, AddHours, AddMinutes, AddSeconds, and AddMilliseconds of DateTime, and DateTimeOfset types. We are using ReflectionHelper.GetMethod to extract the methods from the expression tree.
The BuildHql method of DateTimeFunctionsGenerator is responsible for translating our method call to the HQL expression.
Then we register our custom generator in the HQL generator registry. After that we register our custom registry to the configuration.