我需要在运行时使用给定配置创建表连接。
在这种情况下，我有IQueryable属性，它是root，我需要动态创建连接。
这是我尝试过的。
public class MyDbContext : DbContext
{
public IQueryable<T> AsDynamicQueryable<T>() where T : class
{
var predicate = default(Func<T, bool>); // This is a Dynamically generated predicate
var query = this.Set<T>().Where(predicate).AsQueryable();
// Now here I need to append a JOIN to the above 'query'
// So far, this is what I have done.
var rootType = typeof(T);
var innerType = Type.GetType("This type takes from the configuration");
var innerExpression = this.Set(innerType).AsQueryable();
var paramOne = Expression.Parameter(rootType, "p1");
var paramTwo = Expression.Parameter(innerType, "p2");
var outerKeySelector = Expression.Property(paramOne, "property_one"); //'property_one' is a property of a first parameter which takes from the configuration
var outerKeySelectorExpression = Expression.Lambda(outerKeySelector, paramOne); // (p1)=>p1.property_one
var innerKeySelector = Expression.Property(paramTwo, "property_two"); //'property_two' is a property of a 2nd parameter which takes from the configuration
var innerKeySelectorExpression = Expression.Lambda(innerKeySelector, paramTwo); // (p2)=>p2.property_two
var resultSelector = Expression.Lambda(paramOne, paramOne, paramTwo); // (p1,p2)=>p1
var joinMethod = typeof(Queryable)
.GetMethods()
.First(m => m.Name == "Join" && m.GetParameters().Length == 5)
.MakeGenericMethod(rootType, innerType, typeof(int), rootType);
// 1st Apptempt.
// I'm not sure that I can execute the JOIN method like this.
// But anyway, this gives the below error when I try to execute via taking Count();
// "This method supports the LINQ to Entities infrastructure and is not intended to be used directly from your code."
var newQuery = (IQueryable<T>)joinMethod
.Invoke(
query,
new object[]
{
query,
innerExpression,
outerKeySelectorExpression,
innerKeySelectorExpression,
resultSelector
});
var tt = newQuery.Count(); // Here I just try to execute the expression to check whether it works before I return the Queryable.
// 2nd Attempt
// This also gives the following error when I try to execute via taking Count();
// Unable to create a constant value of type '<type name of the root(T) type>'. Only primitive types or enumeration types are supported in this context.
var joinMethodCallExpression = Expression.Call(
null,
joinMethod,
query.Expression,
innerExpression.Expression,
outerKeySelectorExpression,
innerKeySelectorExpression,
resultSelector);
var xx = this.Set<T>().AsQueryable().Provider.CreateQuery<T>(joinMethodCallExpression);
var te = xx.Count(); // Here I just try to execute the expression to check whether it works before I return the Queryable.
throw new NotImplementedException();
}
}
非常感谢有人能指出正确的方法。
答案 0 :(得分：0)
这是代码。我在代码中添加了我的评论：
public IQueryable<T> AsDynamicQueryable<T>() where T : class
{
// ERROR!!! It should be Expression<Func<T, bool>>
// GetPredicate<T>() is my method to get the predicate. You must
// put here yours. IT must return an Expression<Func<T, bool>>
Expression<Func<T, bool>> predicate = GetPredicate<T>(); // This is a Dynamically generated predicate
// ERROR!!! Don't EVER use AsQueryable(), unless you exactly know
// what you are doing. In this example, your use of AsQueryable<>()
// is hiding the fact that you are executing the Where() LOCALLY,
// because it is a Where(this IEnumerable<>, Func<>) instead of
// being a Where(this IQueryable<>, Expression<>)
// If you want an IQueryable<>, put it in a IQueryable<> variable
IQueryable<T> query = this.Set<T>().Where(predicate);
var rootType = typeof(T);
var innerType = GetAsDynamicQueryableInnerType<T>();
// Same as before! Don't use .AsQueryable(). In this case, use
// IQueryable (non-generic). Note that in this case there was
// no problem with yoru code, so AsQueryable() wasn't doing
// "damage"
IQueryable innerExpression = this.Set(innerType);
var paramOne = Expression.Parameter(rootType, "p1");
var paramTwo = Expression.Parameter(innerType, "p2");
// GetPrimaryKey() is my method to get the property to use.
// it returns a string with the name of the property
string primaryKeyRootType = GetPrimaryKey(rootType);
var outerKeySelector = Expression.Property(paramOne, primaryKeyRootType); //'property_one' is a property of a first parameter which takes from the configuration
var outerKeySelectorExpression = Expression.Lambda(outerKeySelector, paramOne); // (p1)=>p1.property_one
// GetForeignKey() is my method to get the property to use.
// it returns a string with the name of the property
var foreignKeyInnerType = GetForeignKey(innerType, rootType);
var innerKeySelector = Expression.Property(paramTwo, foreignKeyInnerType); //'property_two' is a property of a 2nd parameter which takes from the configuration
var innerKeySelectorExpression = Expression.Lambda(innerKeySelector, paramTwo); // (p2)=>p2.property_two
var resultSelector = Expression.Lambda(paramOne, paramOne, paramTwo); // (p1,p2)=>p1
// Using outerKeySelector.Type as the type of the third parameter
// here. 99% it is typeof(int), but why not make it more generic?
var joinMethod = typeof(Queryable)
.GetMethods()
.First(m => m.Name == "Join" && m.GetParameters().Length == 5)
.MakeGenericMethod(rootType, innerType, outerKeySelector.Type, rootType);
// Queryable.Join is static, so the first parameter must be null!
// Then the parameters to pass to Queryable.Join are the ones you
// where using in the 1st case.
var newQuery = (IQueryable<T>)joinMethod.Invoke(
null,
new object[]
{
query,
innerExpression,
outerKeySelectorExpression,
innerKeySelectorExpression,
resultSelector
});
return newQuery;
}
然后存在一个大问题：即使它可以工作，你也只能获得IQueryable<T>，但加入的结果通常是IQueryable<T+U>。我看到你写了resultSelector = ... (p1,p2)=>p1，但它真的是你想要的吗？