[英]Applying LINQ filters based on a multi-dimensional array
給定實體框架查詢,例如
var query = (from property in _dbContext.Properties
join location in _db.Locations
on property.Id equals location.PropertyId
select new PropertyDetail
{
Url = property.Url,
Type = property.Type,
Title = property.Title,
Continent = location.Continent,
Country = location.Country,
State = location.State,
});
我已應用過濾器,例如:
if (!string.IsNullOrWhitespace(searchFilters.Type))
{
query = query.Where(model => model.Type == searchFilters.Type);
}
if (!string.IsNullOrWhitespace(searchFilters.Title))
{
query = query.Where(model => model.Title.Contains(searchFilters.Title));
}
給出以下多維數組
var locations = new[]
{
new[] {"Africa", "Algeria", ""},
new[] {"Asia", "Hong Kong", ""},
new[] {"Asia", "Singapore", ""},
new[] {"Oceania", "Australia", "New South Wales"},
new[] {"North America", "United States", "California"}
};
如何將“查詢”進一步限制為僅包括與指定位置{大陸,國家,州(可選)}匹配的條目?
這需要SQL中所謂的相關子查詢。 假設它們總是占據相同的位置,您可以使用數組索引器來訪問locations
鋸齒數組中的元素。
query = query.Where(model =>
locations.Any(location =>
location[0] == model.Continent &&
location[1] == model.Country &&
(string.IsNullOrEmpty(location[2]) || location[2] == model.State)));
更新 :由於LINQ to Entities不支持數組索引器,因此您可以將鋸齒狀數組轉換為匿名類型的集合。 (從長遠來看,最好創建一個用於實例化過濾器的類。這比記住每個索引所代表的元素更直觀。)
var locationsTyped =
locations.Select(location => new
{
Continent = location[0],
Country = location[1],
State = location[2],
}).ToArray();
query = query.Where(model =>
locationsTyped.Any(location =>
location.Continent == model.Continent &&
location.Country == model.Country &&
(string.IsNullOrEmpty(location.State) || location.State == model.State)));
不幸的是,LINQ to Entities目前不支持對內存集合的連接,也不支持非原始內存集合的Contains
。 我看到的唯一方法(實際上還有另一個在這里描述的實體框架LINQ獲取所有項目的另一個集合的一部分 ,但現在我認為這更合適)是使用一些表達式構建幫助器來構造OR
過濾器。
例如,使用PredicateUtils
類從linq中的兩個列表到實體where子句之間建立鏈接 ,它可以是這樣的:
首先,添加一個小幫助方法
static Expression<Func<PropertyDetail, bool>> LocationFilter(string value, int index)
{
if (!string.IsNullOrEmpty(value))
{
if (index == 0) return d => d.Continent == value;
if (index == 1) return d => d.Country == value;
if (index == 2) return d => d.State == value;
}
return null;
}
然后使用
var locationsFilter = locations.Select(location => location.Select(LocationFilter)
.Aggregate(PredicateUtils.And)).Aggregate(PredicateUtils.Or);
if (locationsFilter != null)
query = query.Where(locationsFilter);
為了完整性,這里是使用的助手類:
public static class PredicateUtils
{
sealed class Predicate<T>
{
public static readonly Expression<Func<T, bool>> True = item => true;
public static readonly Expression<Func<T, bool>> False = item => false;
}
public static Expression<Func<T, bool>> Null<T>() { return null; }
public static Expression<Func<T, bool>> True<T>() { return Predicate<T>.True; }
public static Expression<Func<T, bool>> False<T>() { return Predicate<T>.False; }
public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> left, Expression<Func<T, bool>> right)
{
if (Equals(left, right)) return left;
if (left == null || Equals(left, True<T>())) return right;
if (right == null || Equals(right, True<T>())) return left;
if (Equals(left, False<T>()) || Equals(right, False<T>())) return False<T>();
var body = Expression.AndAlso(left.Body, right.Body.Replace(right.Parameters[0], left.Parameters[0]));
return Expression.Lambda<Func<T, bool>>(body, left.Parameters);
}
public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> left, Expression<Func<T, bool>> right)
{
if (Equals(left, right)) return left;
if (left == null || Equals(left, False<T>())) return right;
if (right == null || Equals(right, False<T>())) return left;
if (Equals(left, True<T>()) || Equals(right, True<T>())) return True<T>();
var body = Expression.OrElse(left.Body, right.Body.Replace(right.Parameters[0], left.Parameters[0]));
return Expression.Lambda<Func<T, bool>>(body, left.Parameters);
}
static Expression Replace(this Expression expression, Expression source, Expression target)
{
return new ExpressionReplacer { Source = source, Target = target }.Visit(expression);
}
class ExpressionReplacer : ExpressionVisitor
{
public Expression Source;
public Expression Target;
public override Expression Visit(Expression node)
{
return node == Source ? Target : base.Visit(node);
}
}
}
更新:根據評論中的要求,以下是List<Location>
locations
解決方案:
var locationsFilter = locations.Select(location =>
{
var filter = PredicateUtils.Null<PropertyDetail>();
if (!string.IsNullOrEmpty(location.Continent))
filter = filter.And(d => d.Continent == location.Continent);
if (!string.IsNullOrEmpty(location.Country))
filter = filter.And(d => d.Country == location.Country);
if (!string.IsNullOrEmpty(location.State))
filter = filter.And(d => d.State == location.State);
return filter;
}).Aggregate(PredicateUtils.Or);
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