How can I generate an Expression tree that queries an object with List<T> as a property?
Forgive my clumsy explanation, but I have a class that contains a List:
public class Document
{
public int OwnerId { get; set; }
public List<User> Users { get; set; }
public Document() { }
}
public class User
{
public string UserName { get; set; }
public string Department { get; set; }
}
Currently I use PredicateBuilder to perform dynamic queries on my objects. How can I turn the following LINQ statement into an Expression Tree:
var predicate= PredicateBuilder.True<User>();
predicate= predicate.And<User>(user => user.Deparment == "HR");
var deptDocs = documents.AsQueryable()
.Where(doc => doc.Users
.AsQueryable().Count(predicate) > 0)
.ToList();
In other words var deptDocs = documents.HasUserAttributes("Department", "HR").ToList();
. From what I can tell, my issue is not being able to evaluate the user => user.Deparment == "HR"
row by row if I were to issue an Expression.Invoke.
UPDATE:
I think I have made some progress on this:
var predicate = PredicateBuilder.True<User>();
predicate = predicate.And<User>(user => user.Department == "FIN");
Expression<Func<Document, bool>> userSelector =
doc => doc.Users
.AsQueryable()
.Any(predicate);
var docParm = Expression.Parameter(typeof(Document), "appDoc");
var body = Expression.Invoke(userSelector, docParm);
var lambda = Expression.Lambda<Func<Document, bool>>(body, docParm);
var allPredicate = PredicateBuilder.True<Document>();
allPredicate = allPredicate.And<Document>(doc => doc.OwnerId == 1);
allPredicate = allPredicate.And<Document>(lambda);
var hrDocs = this.docs.AsQueryable().Where(allPredicate).ToList();
Is there a more effective way to do this? How can I make an expre开发者_如何学Gossion that creates the predicate - user => user.Department - with generics?
Sounds like you want this toolkit?
I've come up with a somewhat satisfactory solution where I can perform the query with syntax like below:
var predicate = PredicateBuilder.True<Document>();
predicate = predicate.And<Document>(User.SubQuery("UserName", "DAVER"));
predicate = predicate.And<Document>(AdHoc<Document>("OwnerId", 1));
var finDocs = docs.AsQueryable().Where(predicate).ToList();
I have an extension class with this method:
public static Expression<Func<T, bool>> AdHoc<T>
(string columnName, object compValue)
{
// Determine type of parameter
ParameterExpression parameter = Expression.Parameter(typeof(T), "x");
// Target to compare to
Expression property = Expression.Property(parameter, columnName);
// The value to match
Expression constant = Expression.Constant(compValue, compValue.GetType());
Expression equality = Expression.Equal(property, constant);
Expression<Func<T, bool>> predicate =
Expression.Lambda<Func<T, bool>>(equality, parameter);
return predicate;
}
And on my User class I have a static method:
public static Expression<Func<Document, bool>> SubQuery(string property,
string targetValue)
{
var predicate = PredicateBuilder.True<User>();
predicate = predicate.And<User>(Extensions.AdHoc<User>(property, targetValue));
Expression<Func<Document, bool>> userSelector =
doc => doc.Users
.AsQueryable()
.Any(predicate);
var docParm = Expression.Parameter(typeof(Document), "appDoc");
var body = Expression.Invoke(userSelector, docParm);
var docPredicate = PredicateBuilder.True<Document>();
docPredicate = docPredicate.And<Document>(Expression.Lambda<Func<Document, bool>>(body, docParm));
return docPredicate;
}
The drawback is that I've included the Subquery functionality on the User class itself. It gets the job done, but if anyone has any suggestions or a better way to use generics so I don't have to include this static method on my User class I'm eager to hear what you have to say.
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