动态Linq +实体框架：动态选择的日期时间修改

Question

I am trying to find a way to move UTC time to Local before doing a sql grouping. I am using the System.Linq.Dynamic (managed here https://github.com/kahanu/System.Linq.Dynamic ). It works great for doing dynamic selects without having at compile time the required fields. In our case, we store all datetimes in UTC. In this dynamic select, its possible that someone would want to do a groupby on the Hour, year, month, etc. We have to move the data to a local time in this case, to prevent confusion.

Example:

var select = queryable.Select(string.Format("new ({0}, {1})", datetimeColumn, someOtherColumn));

Normally in our tsql or even in entity framework using lambda expressions, you can add in your desired offset. But in the dynamic linq option, it appears that you can't perform any date operations such as DateTime.AddHours(x) or DateTime.Subtract(x) like you could with Linq2Sql. Entity Framework 6 wants you to use DbFunctions.AddHours(x) , etc. However the dynamic linq code, without modification, will not accept the DbFunctions without error.

Example:

var select = queryable.Select(string.Format("new (System.Data.Entity.DbFunctions.AddHours({0},7) as {0}, {1})", datetimeColumn, someOtherColumn));

Returns an error: No property or field 'System' exists in type XXX
(removing the namespace doesn't help).

Using the desired code:

var select = queryable.Select(string.Format("new ({0}.AddHours(7), {1})", datetimeColumn, someOtherColumn));

Results with error: LINQ to Entities does not recognize the method 'System.DateTime AddHours(Double)' method, and this method cannot be translated into a store expression.

I want to have SQL perform the datetime math prior to the groupby. Once the groupby happens, there is no concept of UTC any longer as the user will see the localized result set.

I'm afraid that Ill just to update my github fork with some extensions to support passing in the entity framework extensions, but before I did, wanted to see if anyone else has a solution or idea.

Note: I am not using DateTimeOffset due to possibilities of changing SQL data store technologies.

Answer 1

You can post process the query expression with custom ExpressionVisitor and replace the unsupported methods with their DbFunctions equivalents.

Here is a starting point just to get the idea:

public static class QueryableExtensions
{
    public static IQueryable BindDbFunctions(this IQueryable source)
    {
        var expression = new DbFunctionsBinder().Visit(source.Expression);
        if (expression == source.Expression) return source;
        return source.Provider.CreateQuery(expression);
    }

    class DbFunctionsBinder : ExpressionVisitor
    {
        protected override Expression VisitMethodCall(MethodCallExpression node)
        {
            if (node.Object != null && node.Object.Type == typeof(DateTime))
            {
                if (node.Method.Name == "AddHours")
                {
                    var timeValue = Visit(node.Object);
                    var addValue = Visit(node.Arguments.Single());
                    if (timeValue.Type != typeof(DateTime?)) timeValue = Expression.Convert(timeValue, typeof(DateTime?));
                    if (addValue.Type != typeof(int?)) addValue = Expression.Convert(addValue, typeof(int?));
                    var methodCall = Expression.Call(
                        typeof(DbFunctions), "AddHours", Type.EmptyTypes,
                        timeValue, addValue);
                    return Expression.Convert(methodCall, typeof(DateTime));
                }
            }
            return base.VisitMethodCall(node);
        }
    }
}

and sample usage:

var select = queryable
    .Select(string.Format("new ({0}.AddHours(7), {1})", datetimeColumn, someOtherColumn))
    .BindDbFunctions();