如何 LINQ 按对象列表过滤 dbset

Question

I'm trying to filter dbset of entities of such pseudo-structure (field1, field2, field3) using request of IEnumerable of SomeModel where SomeModel contains pairs (field1, field2) (the same fields subset)

I've tried

var ordersList3 = await _dbContext.MyEntities.
AsNoTracking().
Where(a => request.Contains(new SomeModel() { field1 = a.field1, field2 = a.field2})).
ToListAsync();

but it doesn't work

could you please suggest the correct way of filtering a dbset by list of models containing fields subset?

Answer 1

I do not know if this is nessessary in your case, but sometimes you need to write down the expressions directly. As this can be sort of fun, here you go:

public static Expression<Func<PseudoStructure, bool>> GetPredicate<PseudoStructure>(
    IEnumerable<PseudoStructure> request) {
    var parameter = Expression.Parameter(typeof(PseudoStructure), "o");
    var expression = request.Aggregate(
        (Expression)null,
        (acc, next) => MakeBinary(
            acc, 
            CompareInstances(parameter, next),
            ExpressionType.Or));
    return Expression.Lambda<Func<PseudoStructure, bool>>(expression, parameter);
}

This method generates an Expression<Func<PseudoStructure, bool>> which consists of Expressions checking equality of instances:

private static Expression CompareInstances<PseudoStructure>(
    ParameterExpression parameter,
    PseudoStructure constant) 
        => typeof(PseudoStructure)
        .GetFields()
        .Select(fieldInfo => Expression.Equal(
            Expression.Constant(fieldInfo.GetValue(constant)),
            Expression.Field(parameter, fieldInfo)))
        .Aggregate(
            (Expression)null,
            (expression, binaryExpression) => MakeBinary(expression, binaryExpression, ExpressionType.And)
        );
    }
}

Please note that equality means equality of the values of the fields. MakeBinary is a simple wrapper for Expression.MakeBinary:

private static Expression MakeBinary(Expression left, Expression right, ExpressionType expressionType)
    => left == null || right == null ?
        left ?? right
        : Expression.MakeBinary(expressionType, left, right);

You can use this like so:

public struct Foo
{
    public int A;
    public decimal B;
    public decimal C;
} 

var values = new List<Foo> {
    new Foo {A = 1, B = 2, C = 3},
    new Foo {A = 4, B = 5, C = 6},
    new Foo {A = 7, B = 8, C = 9},
    new Foo {A = 10, B = 11, C = 12}
};

var expression =
    GetPredicate(new[] { 
        new Foo {A = 1, B = 2, C = 3}, 
        new Foo {A = 10, B = 11, C = 12}
    });

var result = values.AsQueryable().Where(expression).ToList();

Answer 2

You get the error because entity framework cannot translate into sql query the expression. This is due to the predicate inside the where clause. The correct implementation would be:

var ordersList3 = await _dbContext.MyEntities.
AsNoTracking().
Where(a => request.Any(r => r.field1 == a.field1 && r.field2 == a.field2})).
ToListAsync();