[英]Linq expression for where condition on entity child
你好,
我如何通過linq表達式構建以下語句以將其轉換為sql?
builder.Where(e => e.SomeChild.Name.ToLower() == "something");
生成builder
是DbContext。 當我直接調用它時,它會像這樣正確解釋
select ... from someTable
inner join childTable on ...
where LOWER(childTable.Name) = @someParam
所以我這樣建立了where條件:
private readonly Expression<Func<TEntity, TProperty>> _property;
private Expression<Func<TEntity, bool>> insensitiveEqualityPredicate(string formula)
{
var parameter = Expression.Parameter(typeof(TEntity));
var property = (PropertyInfo)((MemberExpression)_property.Body).Member;
var propertyParameter = Expression.Parameter(property.DeclaringType);
var lowerExpression = Expression.Call(
Expression.Invoke(_property, propertyParameter),
typeof(string).GetMethods().Where(m => m.Name == "ToLower" && m.GetParameters().ToList().Count == 0).FirstOrDefault()
);
return Expression.Lambda<Func<TEntity, bool>>(
Expression.Equal(
lowerExpression,
Expression.Constant(formula.ToLower(), typeof(string))
),
parameter
);
}
它像魅力一樣工作:
_property = e => e.Name;
...
builder.Where(insensitiveEqualityPredicate("whatever"));
但不適用於兒童的情況:
_property = e => e.SomeChild.Name;
...
builder.Where(insensitiveEqualityPredicate("whatever"));
因為:
The LINQ expression 'Param_0.Name.Equal("whatever")' could not be translated and will be evaluated locally.
正確的方法是什么?
先感謝您。
據我了解,您有一個類型為Expression<Func<TEntity, TProperty>>
的字段_property
(在這種情況下,實際上是Expression<Func<TEntity, string>>
),該e => e.Name
已使用lambda表達式(例如e => e.Name
)進行了初始化。 , e => e.SomeChild.Name
等。
為了構建所需的謂詞,您可以重用_property
表達式參數,並將Body
屬性用作新表達式的基礎:
private readonly Expression<Func<TEntity, TProperty>> _property;
private Expression<Func<TEntity, bool>> insensitiveEqualityPredicate(string formula)
{
return Expression.Lambda<Func<TEntity, bool>>(
Expression.Equal(
Expression.Call(_property.Body, "ToLower", Type.EmptyTypes),
Expression.Constant(formula.ToLower(), typeof(string))
),
_property.Parameters
);
}
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