[英]How to LINQ filter dbset by list of objects
我正在尝试使用 SomeModel 的 IEnumerable 请求过滤此类伪结构(field1,field2,field3)的实体的数据库集,其中 SomeModel 包含对(field1,field2)(相同的字段子集)
我试过了
var ordersList3 = await _dbContext.MyEntities.
AsNoTracking().
Where(a => request.Contains(new SomeModel() { field1 = a.field1, field2 = a.field2})).
ToListAsync();
但它不起作用
您能否建议通过包含字段子集的模型列表过滤数据库集的正确方法?
我不知道这是否对你来说是必要的,但有时你需要直接写下表达式。 因为这可能很有趣,所以这里是 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);
}
此方法生成一个 Expression<Func<PseudoStructure, bool>> ,其中包含检查实例相等性的表达式:
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)
);
}
}
请注意,相等意味着字段值的相等。 MakeBinary 是 Expression.MakeBinary 的简单包装器:
private static Expression MakeBinary(Expression left, Expression right, ExpressionType expressionType)
=> left == null || right == null ?
left ?? right
: Expression.MakeBinary(expressionType, left, right);
您可以像这样使用它:
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();
您收到错误是因为实体框架无法转换为 sql 查询表达式。 这是由于 where 子句中的谓词。 正确的实现是:
var ordersList3 = await _dbContext.MyEntities.
AsNoTracking().
Where(a => request.Any(r => r.field1 == a.field1 && r.field2 == a.field2})).
ToListAsync();
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