Use LinqKit PredicateBuilder for related model (EF Core)

问题

I want to use LinqKit's PredicateBuilder and pass the predicate into .Any method for related model.

So I want to build a predicate:

var castCondition = PredicateBuilder.New<CastInfo>(true);

if (movies != null && movies.Length > 0)
{
    castCondition = castCondition.And(c => movies.Contains(c.MovieId));
}
if (roleType > 0)
{
    castCondition = castCondition.And(c => c.RoleId == roleType);
}

And then use it to filter model that has relation to model in predicate:

IQueryable<Name> result = _context.Name.AsExpandable().Where(n => n.CastInfo.Any(castCondition));
return await result.OrderBy(n => n.Name1).Take(25).ToListAsync();

But this causes a System.NotSupportedException: Could not parse expression 'n.CastInfo.Any(Convert(__castCondition_0, Func``2))': The given arguments did not match the expected arguments: Object of type 'System.Linq.Expressions.UnaryExpression' cannot be converted to type 'System.Linq.Expressions.LambdaExpression'.

I saw similar question and answer there suggests to use .Compile. Or one more question that build an extra predicate.

So I tried to use extra predicate

var tp = PredicateBuilder.New<Name>(true);
tp = tp.And(n => n.CastInfo.Any(castCondition.Compile()));
IQueryable<Name> result = _context.Name.AsExpandable().Where(tp);

Or use compile directly

IQueryable<Name> result = _context.Name.AsExpandable().Where(n => n.CastInfo.Any(castCondition.Compile()));

But I have an error about Compile: System.NotSupportedException: Could not parse expression 'n.CastInfo.Any(__Compile_0)'

So is it possible to convert the result from PredicateBuilder to pass into Any?

Note: I was able to build the desired behavior combining expressions, but I don't like that I need extra variables.

System.Linq.Expressions.Expression<Func<CastInfo,bool>> castExpression = (c => true);
if (movies != null && movies.Length > 0)
{
    castExpression = (c => movies.Contains(c.MovieId));
}
if (roleType > 0)
{
    var existingExpression = castExpression;
    castExpression = c => existingExpression.Invoke(c) && c.RoleId == roleType;
}
IQueryable<Name> result = _context.Name.AsExpandable().Where(n => n.CastInfo.Any(castExpression.Compile()));
return await result.OrderBy(n => n.Name1).Take(25).ToListAsync();

So I assume I just miss something about builder.

Update about versions: I use dotnet core 2.0 and LinqKit.Microsoft.EntityFrameworkCore 1.1.10

回答1:

Looking at the code, one will assume that the type of castCondition variable is Expression<Func<CastInfo, bool>> (as it was in earlier versions of PredicateBuilder).

But if that was the case, then n.CastInfo.Any(castCondition) should not even compile (assuming CastInfo is a collection navigation property, so the compiler will hit Enumerable.Any which expects Func<CastInfo, bool>, not Expression<Func<CastInfo, bool>>). So what's going on here?

In my opinion, this is a good example of C# implicit operator abuse. The PredicateBuilder.New<T> method actually returns a class called ExpressionStarter<T>, which has many methods emulating Expression, but more importantly, has implicit conversion to Expression<Func<T, bool>> and Func<CastInfo, bool>. The later allows that class to be used for top level Enumerable / Queryable methods as replacement of the respective lambda func/expression. However, it also prevents the compile time error when used inside the expression tree as in your case - the complier emits something like n.CastInfo.Any((Func<CastInfo, bool>)castCondition) which of course causes exception at runtime.

The whole idea of LinqKit AsExpandable method is to allow "invoking" expressions via custom Invoke extension method, which then is "expanded" in the expression tree. So back at the beginning, if the variable type was Expression<Func<CastInfo, bool>>, the intended usage is:

_context.Name.AsExpandable().Where(n => n.CastInfo.Any(c => castCondition.Invoke(c)));

But now this doesn't compile because of the reason explained earlier. So you have to convert it first to Expression<Func<T, bool> outside of the query:

Expression<Func<CastInfo, bool>> castPredicate = castCondition;

and then use

_context.Name.AsExpandable().Where(n => n.CastInfo.Any(c => castPredicate.Invoke(c)));

_context.Name.AsExpandable().Where(n => n.CastInfo.Any(castPredicate.Compile()));

To let compiler infer the expression type, I would create a custom extension method like this:

using System;
using System.Linq.Expressions;

namespace LinqKit
{
    public static class Extensions
    {
        public static Expression<Func<T, bool>> ToExpression<T>(this ExpressionStarter<T> expr) => expr;
    }
}

and then simply use

var castPredicate = castCondition.ToExpression();

It still has to be done outside of the query, i.e. the following does not work:

_context.Name.AsExpandable().Where(n => n.CastInfo.Any(c => castCondition.ToExpression().Invoke(c)));

来源：https://stackoverflow.com/questions/46706255/use-linqkit-predicatebuilder-for-related-model-ef-core

标签

linq

.net-core

entity-framework-core

linqkit