[英]How to LINQ filter dbset by list of objects
我正在嘗試使用 SomeModel 的 IEnumerable 請求過濾此類偽結構(field1,field2,field3)的實體的數據庫集,其中 SomeModel 包含對(field1,field2)(相同的字段子集)
我試過了
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
您能否建議通過包含字段子集的模型列表過濾數據庫集的正確方法?
我不知道這是否對你來說是必要的,但有時你需要直接寫下表達式。 因為這可能很有趣,所以這里是 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);
}
此方法生成一個 Expression<Func<PseudoStructure, bool>> ,其中包含檢查實例相等性的表達式:
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)
);
}
}
請注意,相等意味着字段值的相等。 MakeBinary 是 Expression.MakeBinary 的簡單包裝器:
private static Expression MakeBinary(Expression left, Expression right, ExpressionType expressionType)
=> left == null || right == null ?
left ?? right
: Expression.MakeBinary(expressionType, left, right);
您可以像這樣使用它:
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();
您收到錯誤是因為實體框架無法轉換為 sql 查詢表達式。 這是由於 where 子句中的謂詞。 正確的實現是:
var ordersList3 = await _dbContext.MyEntities.
AsNoTracking().
Where(a => request.Any(r => r.field1 == a.field1 && r.field2 == a.field2})).
ToListAsync();
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