背景
我的客户想要一种通过字段（字符串）、值（字符串）和比较（枚举）值的数组发送的方法，以便检索他们的数据。
public class QueryableFilter {
public string Name { get; set; }
public string Value { get; set; }
public QueryableFilterCompareEnum? Compare { get; set; }
}
我和我的公司以前从未尝试过这样的事情，所以我的团队有责任想出一个可行的解决方案。这是经过一周左右的研究得出解决方案的结果。
什么有效：第 1 部分
我创建了一个能够从我们的表Classroom中检索数据的服务。数据的检索是在 Entity Framework Core 中通过 LINQ-to-SQL 完成的。如果过滤器中提供的字段之一对于Classroom不存在但对于其相关组织确实存在（客户也希望能够在组织地址中搜索）并且具有可导航的，我在下面编写的方式有效财产。
public async Task<IEnumerable<IExportClassroom>> GetClassroomsAsync(
IEnumerable<QueryableFilter> queryableFilters = null) {
var filters = queryableFilters?.ToList();
IQueryable<ClassroomEntity> classroomQuery = ClassroomEntity.All().AsNoTracking();
// The organization table may have filters searched against it
// If any are, the organization table should be inner joined to all filters are used
IQueryable<OrganizationEntity> organizationQuery = OrganizationEntity.All().AsNoTracking();
var joinOrganizationQuery = false;
// Loop through the supplied queryable filters (if any) to construct a dynamic LINQ-to-SQL queryable
if (filters?.Count > 0) {
foreach (var filter in filters) {
try {
classroomQuery = classroomQuery.BuildExpression(filter.Name, filter.Value, filter.Compare);
} catch (ArgumentException ex) {
if (ex.ParamName == "propertyName") {
organizationQuery = organizationQuery.BuildExpression(filter.Name, filter.Value, filter.Compare);
joinOrganizationQuery = true;
} else {
throw new ArgumentException(ex.Message);
}
}
}
}
// Inner join the classroom and organization queriables (if necessary)
var query = joinOrganizationQuery
? classroomQuery.Join(organizationQuery, classroom => classroom.OrgId, org => org.OrgId, (classroom, org) => classroom)
: classroomQuery;
query = query.OrderBy(x => x.ClassroomId);
IEnumerable<IExportClassroom> results = await query.Select(ClassroomMapper).ToListAsync();
return results;
}
什么有效：第 2 部分
代码中存在的BuildExpression是我这样创建的（有扩展空间）。
public static IQueryable<T> BuildExpression<T>(this IQueryable<T> source, string columnName, string value, QueryableFilterCompareEnum? compare = QueryableFilterCompareEnum.Equal) {
var param = Expression.Parameter(typeof(T));
// Get the field/column from the Entity that matches the supplied columnName value
// If the field/column does not exists on the Entity, throw an exception; There is nothing more that can be done
MemberExpression dataField;
try {
dataField = Expression.Property(param, propertyName);
} catch (ArgumentException ex) {
if (ex.ParamName == "propertyName") {
throw new ArgumentException($"Queryable selection does not have a \"{propertyName}\" field.", ex.ParamName);
} else {
throw new ArgumentException(ex.Message);
}
}
ConstantExpression constant = !string.IsNullOrWhiteSpace(value)
? Expression.Constant(value.Trim(), typeof(string))
: Expression.Constant(value, typeof(string));
BinaryExpression binary = GetBinaryExpression(dataField, constant, compare);
Expression<Func<T, bool>> lambda = (Expression<Func<T, bool>>)Expression.Lambda(binary, param)
return source.Where(lambda);
}
private static Expression GetBinaryExpression(MemberExpression member, ConstantExpression constant, QueryableFilterCompareEnum? comparisonOperation) {
switch (comparisonOperation) {
case QueryableFilterCompareEnum.NotEqual:
return Expression.Equal(member, constant);
case QueryableFilterCompareEnum.GreaterThan:
return Expression.GreaterThan(member, constant);
case QueryableFilterCompareEnum.GreaterThanOrEqual:
return Expression.GreaterThanOrEqual(member, constant);
case QueryableFilterCompareEnum.LessThan:
return Expression.LessThan(member, constant);
case QueryableFilterCompareEnum.LessThanOrEqual:
return Expression.LessThanOrEqual(member, constant);
case QueryableFilterCompareEnum.Equal:
default:
return Expression.Equal(member, constant);
}
}
}
问题/解决我的问题
虽然课堂和组织上的内部连接有效，但我宁愿不必引入第二个实体集来检查可导航的值。如果我输入一个城市作为我的过滤器名称，通常我会这样做：
classroomQuery = classroomQuery.Where(x => x.Organization.City == "Atlanta");
这在这里真的行不通。
为了得到我想要的东西，我尝试了几种不同的方法：
- 将返回 Func<T, bool> 的已编译函数，但在通过 LINQ-to-SQL 传递时，查询不包含它。
- 我将其更改为 Expression<Func<T, bool>>，但我的 return 没有以我尝试实现它的方式返回 bool，所以这不起作用。
- 我切换了实现导航属性的方式，但我的任何函数都不能正确读取该值。
基本上，有没有什么方法可以让我从 Entity Framework Core 的 LINQ-to-SQL 中实现以下功能？也欢迎其他选择。
classroomQuery = classroomQuery.Where(x => x.Organization.BuildExpression(filter.Name, filter.Value, filter.Compare));
编辑01：
当使用没有动态构建器的表达式时，如下所示：
IQueryable<ClassroomEntity>classroomQuery = ClassroomEntity.Where(x => x.ClassroomId.HasValue).Where(x => x.Organization.City == "Atlanta").AsNoTracking();
调试内容如下：
.Call Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.AsNoTracking(.Call System.Linq.Queryable.Where(
.Call System.Linq.Queryable.Where(
.Constant<Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryable`1[ClassroomEntity]>(Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryable`1[ClassroomEntity]),
'(.Lambda #Lambda1<System.Func`2[ClassroomEntity,System.Boolean]>)),
'(.Lambda #Lambda2<System.Func`2[ClassroomEntity,System.Boolean]>)))
.Lambda #Lambda1<System.Func`2[ClassroomEntity,System.Boolean]>(ClassroomEntity $x)
{
($x.ClassroomId).HasValue
}
.Lambda #Lambda2<System.Func`2[ClassroomEntity,System.Boolean]>(ClassroomEntity $x)
{
($x.Organization).City == "Bronx"
}
我尝试使用动态构建器来获取课堂老师，这给了我一个调试：
.Lambda #Lambda3<System.Func`2[ClassroomEntity,System.Boolean]>(ClassroomEntity $var1)
{
$var1.LeadTeacherName == "Sharon Candelariatest"
}
仍然无法弄清楚如何将($var1.Organization)作为我正在读取的实体。