问题描述
我对具有多个链接表的数据库进行LINQ查询,并且需要根据输入返回(选择)不同的字段.
ClassA has ParamA, ParamB, and ICollection<ClassB>ClassBs
ClassB has ParamD, ParamE
Linq查询的部分:
.Select(c => new ClassA()
{
ParamA = c.ParamA,
ParamB = c.ParamB,
ClassBs = c.ClassBs.Select(p => new ClassB()
{
ParamD = p.ParamD,
ParamE = p.ParamE
}).ToList()
}).ToList();
在某些电话上,我只需要ParamA和ParamE.在其他调用上,可能是ParamB和ParamE.我已经可以使用Expressions来完成这项工作,但不能用于ICollection.我尝试使用Dynamic.Linq.Core,但在SELECT中找不到用于更新的任何示例.我更喜欢使用MemberExpressions ...
[更新]更多上下文:ClassA和ClassB本质上是指向SQL表的EF模型. ClassA与ClassB有一对多的关系,因此,为什么我要以这种方式查询它们.班级A是学生记录(姓名,地址,住所等),班级B是测试记录,学生可以进行多个测试,每个测试都有日期,成绩,等级,最高分数得分,最低分数得分等等. >
我并不总是想要两个表的所有字段,因为可能有100万条记录,因此,为什么我只选择SELECT特定查询和操作所需的内容.
这可能有点冗长,但是表达式可以做到这一点,例如:
var typeClassA = typeof(ClassA);
var typeClassB = typeof(ClassB);
var parameterExpressionP = Expression.Parameter(typeClassB, "p");
var newExpression = Expression.New(typeClassB);
var paramDPropertyExpression = Expression.Property(parameterExpressionP, "ParamD");
var paramDMemberBinding = Expression.Bind(typeClassB.GetProperty("ParamD"), paramDPropertyExpression);
var paramEPropertyExpression = Expression.Property(parameterExpressionP, "ParamE");
var paramEMemberBinding = Expression.Bind(typeClassB.GetProperty("ParamE"), paramEPropertyExpression);
var memberInitExpression = Expression.MemberInit(
newExpression,
paramDMemberBinding, paramEMemberBinding);
var projectionExpression = Expression.Lambda<Func<ClassB, ClassB>>(memberInitExpression, parameterExpressionP);
var parameterExpressionC = Expression.Parameter(typeClassA, "c");
var selectParamExpression = Expression.Property(parameterExpressionC, "ClassBs");
var selectExpression = Expression.Call(
typeof(Enumerable),
nameof(Enumerable.Select),
new[] { typeClassB, typeClassB },
selectParamExpression, projectionExpression);
var toListExpression = Expression.Call(
typeof(Enumerable),
nameof(Enumerable.ToList),
new[] { typeClassB },
selectExpression);
这将创建类似以下内容的表达式:
c.ClassBs.Select(p => new ClassB() {ParamD = p.ParamD, ParamE = p.ParamE}).ToList()
I have a LINQ query to a database, with multiple linked tables, and need to return (SELECT) different fields depending on inputs.
ClassA has ParamA, ParamB, and ICollection<ClassB>ClassBs
ClassB has ParamD, ParamE
Portion of the Linq query:
.Select(c => new ClassA()
{
ParamA = c.ParamA,
ParamB = c.ParamB,
ClassBs = c.ClassBs.Select(p => new ClassB()
{
ParamD = p.ParamD,
ParamE = p.ParamE
}).ToList()
}).ToList();
On some calls, I'll want ParamA and ParamE only. On other calls, perhaps ParamB and ParamE. I've been able to make this work using Expressions but not for the ICollection. I attempted using Dynamic.Linq.Core, but couldn't find any examples for newing up in the SELECT. I would prefer to do this with MemberExpressions...
[UPDATE]A bit more context: ClassA and ClassB are essentially EF Models pointing to SQL tables. ClassA has a one-to-many relationship to ClassB, thus, why I query them in this fashion. ClassA would be a Student record (name, address, home, etc), ClassB would be a Test record, where student may have more than one test, each test has date taken, grade, highest_score_of_class, lowest_score_of_class, and many more.
I don't always want all the fields of both tables, as there may be 1 million records, thus, why I prefer to SELECT only what is needed for the specific query and operation requested.
This may be a bit verbose, but Expressions can get that way, for example:
var typeClassA = typeof(ClassA);
var typeClassB = typeof(ClassB);
var parameterExpressionP = Expression.Parameter(typeClassB, "p");
var newExpression = Expression.New(typeClassB);
var paramDPropertyExpression = Expression.Property(parameterExpressionP, "ParamD");
var paramDMemberBinding = Expression.Bind(typeClassB.GetProperty("ParamD"), paramDPropertyExpression);
var paramEPropertyExpression = Expression.Property(parameterExpressionP, "ParamE");
var paramEMemberBinding = Expression.Bind(typeClassB.GetProperty("ParamE"), paramEPropertyExpression);
var memberInitExpression = Expression.MemberInit(
newExpression,
paramDMemberBinding, paramEMemberBinding);
var projectionExpression = Expression.Lambda<Func<ClassB, ClassB>>(memberInitExpression, parameterExpressionP);
var parameterExpressionC = Expression.Parameter(typeClassA, "c");
var selectParamExpression = Expression.Property(parameterExpressionC, "ClassBs");
var selectExpression = Expression.Call(
typeof(Enumerable),
nameof(Enumerable.Select),
new[] { typeClassB, typeClassB },
selectParamExpression, projectionExpression);
var toListExpression = Expression.Call(
typeof(Enumerable),
nameof(Enumerable.ToList),
new[] { typeClassB },
selectExpression);
This will create an expression something like:
c.ClassBs.Select(p => new ClassB() {ParamD = p.ParamD, ParamE = p.ParamE}).ToList()
这篇关于在运行时动态选择LINQ SELECT字段的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持!