我的一个Join键选择器如下所示:
x => x.A + "-" + x.B
NHibernate使"-"
成为额外的参数。此参数获取SQL类型nvarchar
,因此整个语句在SQL Server上从varchar
转换为nvarchar
。
问题是,如果查询的列的类型为varchar
而不是nvarchar
,则SQL Server会出现大问题。这是因为该列的类型不是参数,因此无法使用索引。
我无法更改列的类型,所以我需要在转换lambdas时以某种方式定义NHibernate 应该使用varchar作为字符串文字。
有什么办法吗?
在Oskar Berggren的帮助下,我设置了这个课程:
public static class VarcharFix
{
/// This method returns its argument and is a no-op in C#.
/// It's presence in a Linq expression sends a message to the NHibernate Linq Provider.
public static string AsVarchar(string s)
{
return s;
}
}
public class MyHqlIdent : HqlExpression
{
internal MyHqlIdent(IASTFactory factory, string ident)
: base(HqlSqlWalker.IDENT, ident, factory)
{
}
internal MyHqlIdent(IASTFactory factory, System.Type type)
: base(HqlSqlWalker.IDENT, "", factory)
{
if (IsNullableType(type))
{
type = ExtractUnderlyingTypeFromNullable(type);
}
switch (System.Type.GetTypeCode(type))
{
case TypeCode.Boolean:
SetText("bool");
break;
case TypeCode.Int16:
SetText("short");
break;
case TypeCode.Int32:
SetText("integer");
break;
case TypeCode.Int64:
SetText("long");
break;
case TypeCode.Decimal:
SetText("decimal");
break;
case TypeCode.Single:
SetText("single");
break;
case TypeCode.DateTime:
SetText("datetime");
break;
case TypeCode.String:
SetText("string");
break;
case TypeCode.Double:
SetText("double");
break;
default:
if (type == typeof(Guid))
{
SetText("guid");
break;
}
if (type == typeof(DateTimeOffset))
{
SetText("datetimeoffset");
break;
}
throw new NotSupportedException(string.Format("Don't currently support idents of type {0}", type.Name));
}
}
private static System.Type ExtractUnderlyingTypeFromNullable(System.Type type)
{
return type.GetGenericArguments()[0];
}
// TODO - code duplicated in LinqExtensionMethods
private static bool IsNullableType(System.Type type)
{
return (type.IsGenericType && type.GetGenericTypeDefinition() == typeof(Nullable<>));
}
}
public class MyHqlCast : HqlExpression
{
public MyHqlCast(IASTFactory factory, IEnumerable<HqlTreeNode> children)
: base(HqlSqlWalker.METHOD_CALL, "method", factory, children)
{
}
public static MyHqlCast Create(IASTFactory factory, HqlExpression expression, string targetType)
{
return new MyHqlCast(factory,
new HqlTreeNode[]
{
new MyHqlIdent(factory, "cast"),
new HqlExpressionList(factory, expression,
new MyHqlIdent(factory, targetType))
});
}
}
public class MyBaseHqlGeneratorForMethod : BaseHqlGeneratorForMethod
{
public MyBaseHqlGeneratorForMethod()
: base()
{
SupportedMethods = new MethodInfo[] { typeof(VarcharFix).GetMethod("AsVarchar") };
}
public override HqlTreeNode BuildHql(MethodInfo method, System.Linq.Expressions.Expression targetObject, System.Collections.ObjectModel.ReadOnlyCollection<System.Linq.Expressions.Expression> arguments, HqlTreeBuilder treeBuilder, global::NHibernate.Linq.Visitors.IHqlExpressionVisitor visitor)
{
return MyHqlCast.Create(new ASTFactory(new ASTTreeAdaptor()),
visitor.Visit(targetObject).AsExpression(),
"varchar");
}
}
public class ExtendedLinqtoHqlGeneratorsRegistry : DefaultLinqToHqlGeneratorsRegistry
{
public ExtendedLinqtoHqlGeneratorsRegistry()
{
this.Merge(new MyBaseHqlGeneratorForMethod());
}
}
现在它还没有用,但我看到了光;)
var query = aQueryable
.Join(bQueryable,
x => x.AB, x => x.A + VarcharFix.AsVarchar("-") + x.B,
(head, middle) => new ...)
当"-".AsVarchar()
优化为 "-"
时,我们需要虚拟参数,这不能像"-".AsVarchar(x.A)
那样进行优化 - 这样Linq-extension开始了!
var query = aQueryable
.Join(bQueryable,
x => x.AB, x => x.A + "-".AsVarchar(x.A) + x.B,
(head, middle) => new ...)
答案 0 :(得分:1)
可能有多种方法可以做到这一点,但这里有一个:
创建您自己的方法,例如:
/// This method returns its argument and is a no-op in C#.
/// It's presence in a Linq expression sends a message to the NHibernate Linq Provider.
public static string AsVarchar(string s)
{
return s;
}
还创建一个表示HQL表达式片段的类:
public class MyHqlCast : HqlExpression
{
private MyHqlCast(IASTFactory factory, IEnumerable<HqlTreeNode> children)
: base(HqlSqlWalker.METHOD_CALL, "method", factory, children)
{
}
public static MyHqlCast Create(IASTFactory factory, HqlExpression expression,
string targetType)
{
return new MyHqlCast(factory,
new [] {
new HqlIdent(factory, "cast")),
new HqlExpressionList(factory, expression,
new HqlIdent(factory, targetType)),
});
}
}
然后从BaseHqlGeneratorForMethod派生一个类。在其构造函数中,将SupportedMethods属性设置为AsVarchar()方法。重写BuildHql()方法。它应该输出等效于强制转换的HQL强制转换构造(@param as varchar)。通常你会在treeBuilder参数上使用Cast()方法,但不幸的是,它只接受一个System.Type,这对于这种情况来说还不够好。而是创建并返回MyHqlCast的实例:
return MyHqlCast.Create(new ASTFactory(new ASTTreeAdaptor()),
visitor.Visit(arguments[0]).AsExpression(),
"varchar");
然后,需要通过派生自DefaultLinqToHqlGeneratorsRegistry来注册BaseHqlGeneratorForMethod的实现。调用this.Merge(new MyGenerator());在构造函数中。然后通过
注册您的注册表类型nhibernateConfiguration.LinqToHqlGeneratorsRegistry<MyRegistry>();