I have a repository class which takes QueryConstraints object as a parameter to query the data store and return an IEnumerable of desired entity. The repository is supposed to prevent queries which may take too long to process. The QueryConstraints is as follow:
public class QueryConstraints<T>
{
public Expression<Func<T, bool>> WhereClause { get; }
public Expression<Func<T, bool>> SortClause { get; }
public int Skip {get;}
public int Take {get;}
...
some other fields
}
When a client creates a QueryConstraints it should assign expressions to WhereClause and SortClause properties and pass the object to the Find() method of the repository where the expressions are to be inspected and processed.
My question is:
I know that it is possible to use expression tree to inspect the body of the expression. However I don't know how to do it exactly to achieve my purpose. For example I want to prevent a query which has no index available for its fields. How should i do that?
Related
I am trying to implement OrderBy and ThenBy in a different way to hide lambda expression from OrderBy and ThenBy extension methods. These extension methods accept classes which implement IOrderSpecification:
public class PersonOrderByAgeSpecification : OrderSpecification<Person>
{
public PersonOrderByAgeSpecification(Sort direction= Sort.Ascending) : base(direction)
{
}
public override Expression<Func<Person, IComparable>> AsExpression()
{
return personOrder => personOrder.Age;
}
}
And the usage:
var orderSpecification = new PersonOrderByAgeSpecification(Sort.Ascending);
var sortedPeople= _dbContext.People.OrderBy(orderSpecification);
It works fine when the property type in AsExpression() is just string. For example:
public override Expression<Func<Person, IComparable>> AsExpression()
{
return personOrder => personOrder.FirstName;
}
Otherwise I would get this error: (Does not work with integer or bool)
InvalidOperationException: Null TypeMapping in Sql Tree
Microsoft.EntityFrameworkCore.Relational.Query.Pipeline.RelationalSqlTranslatingExpressionVisitor+SqlTypeMappingVerifyingExpressionVisitor.VisitExtension(Expression
node)
The source code is available here
I appreciate any help.
First off, you are using preview (beta) software, which is expected to have issues.
But the main problem is that LINQ ordering methods have second generic type argument TKey, which you are hiding behind IComparable, which for value types causes a hidden cast inside the expression.
Apart from unnecessary boxing, this is not a problem for LINQ to Objects provider because it simply compiles and executes a delegate from the lambda expression. However other IQueryable providers usually need to translate the expression to something else (usually SQL). Most of them identify such casts (Expression.Convert) and remove them during the processing. Apparently EF 3.0 preview you are using doesn't, hence the exception.
You can avoid such issues by eliminating the hidden casts yourself. It's possible to do that with expression manipulation, but the easiest is to introduce the second generic type argument to your base abstract class:
public abstract class OrderSpecification<T, TKey> : IOrderSpecification<T>
and change the abstract method signature to
public abstract Expression<Func<T, TKey>> AsExpression();
The implementation, interface and everything else except the concrete classes will remain as is.
Now all you need is to specify the actual key type in the inherited class and change the AsExpression override signature. For instance:
public class Person
{
public int Id { get; set; }
public string Name { get; set; }
public int Age { get; set; }
}
public class PersonAgeOrderSpecification : OrderSpecification<Person, int>
{
public PersonAgeOrderSpecification(Sort direction) : base(direction) { }
public override Expression<Func<Person, int>> AsExpression()
{
return person => person.Age;
}
}
and everything will be fine.
I am retrieving some tuples from a database that are mapped to entity classes by means of Entity Framework.
For these entities, I have a key selector function (supplied at runtime by other developers) that I would like to pass to Queryable.OrderBy. The key selector function is provided upon "registration" of the entity type in my system - which happens by means of a method that looks roughly like this:
public void RegisterEntity<TEntity, TKey>(string entityName, TKey defaultKey, Func<TEntity, TKey> keySelectorFunc)
I would like to execute this OrderBy call before materializing the results to entity objects (i.e. in such a way that the OrderBy call still gets translated to SQL under the hood).
The problem is that the entities have composite keys, and thus, the key selector function will return a custom object instantiated in the function. You can imagine it like this:
var keySelectorFunc = e => new CustomKey(e.Value1, e.Value2);
As usual, Entity Framework does not like this (the usual "Only parameterless constructors and initializers are supported in LINQ to Entities" error).
Is there any way to use such a custom key selector function to return a custom key? Do I have to resort to anonymous classes? Or should I move the OrderBy call to a place after I have left the LINQ-to-Entities world?
In this particular case it would be easy to use Sort method of Generic List.
https://msdn.microsoft.com/en-us/library/3da4abas(v=vs.110).aspx
Sort method requires the type of the list to implement IComparable interface and it uses the implementation of CompareTo method from IComparable interface. Otherwise implementation of IComparer also can be passed to this method.
So if your entity class is already implemeting IComparable interface then this should surely work for you. You will have to to .ToList() on the IQueryable result of course before you can call the Sort method on it.
public class Category : IComparable<Category>
{
public int CategoryId { get; internal set; }
public string CategoryName { get; internal set; }
public int CompareTo(Category x)
{
return String.Compare(x.CategoryName, this.CategoryName, StringComparison.InvariantCulture);
}
}
List<Category> categories = new List<Category>();
categories.Add(new Category {CategoryName = "Cate1"});
categories.Add(new Category {CategoryName = "Cate2"});
categories.Sort();
foreach (var cat in categories)
{
Console.WriteLine(cat.CategoryName);
}
This displays me category names in reverse order based on the comparison logic I have written in the CompareTo method of Category Class.
In this case I think the best way is use a custom ExtensionMethod to avoid any overhead of coding or unnecessary complexity to do that.
See if it implementation can help you.
First we create your customkey class that is responsable to create the statement expressions:
class CustomKey
{
public CustomKey(params string[] value)
{
if(!value.Any())
throw new InvalidOperationException("Select at least one Property for this operation");
Values = new List<string>();
Values.AddRange(value);
}
private List<string> Values { get; set; }
// this method run throughout all property configured to create the expressions
public void ForEachProperty<TSource, TKey>(Action<Expression<Func<TSource, TKey>>, bool> method)
{
bool firstItem = true;
Values.ForEach(f =>
{
var expression = CreateExpression<TSource, TKey>(f);
method(expression, firstItem);
firstItem = false;
});
}
// this method is responsable to create each expression
Expression<Func<TSource, TKey>> CreateExpression<TSource, TKey>(string property)
{
var parameter = Expression.Parameter(typeof(TSource), "x");
var member = typeof(TSource).GetMember(property).FirstOrDefault();
Expression body = Expression.MakeMemberAccess(parameter, member);
return Expression.Lambda<Func<TSource, TKey>>(Expression.Convert(body, typeof(object)), parameter);
}
}
After that we create your custom ExtesionMethod, somethink like that:
public static class OrderByExtensionClass
{
// instead of try passing an expression, we pass our CustomKey object with the columns to sort.
// than this method create the apropriate OrderBy Expression statement
public static IOrderedQueryable<TSource> OrderBy<TSource>(this IQueryable<TSource> source, CustomKey customKey)
{
// the parameter isFirst is just to control where we are to build the expression
customKey.ForEachProperty<TSource, object>((expression, isFirst) =>
{
if (isFirst)
source = source.OrderBy(expression);
else
source = ((IOrderedQueryable<TSource>)source).ThenBy(expression);
});
return ((IOrderedQueryable<TSource>)source);
}
}
After that we just do:
CustomKey custom = new CustomKey("Name", "Age");
myEntityContext.People.OrderBy(custom).ToList()
I hope it can help you.
Part of the problem, I think, is that OrderBy wouldn't know what to do with a complex type. SQL Server knows how to order by primitive types, but that's about it. You would have to do something like ...OrderBy(x=>x.Field1).ThenBy(x=>x.Field2). You could write an extension method that takes the key, extracts the property names from the key, and builds the .OrderBy().ThenBy() expression, as long as you know what the key will be before executing the query. Otherwise yeah, you may have to materialize the results before ordering.
I am currently using one of the many repository patterns available online to perform CRUD operations with EF6. I am happy to use it as is but I have recently been handed a few legacy projects that have database tables with a very high number of columns. I would like a way to make my application as well as future applications smoother by devising a way to select only a subset of columns.
Current method.
public virtual TEntity Get(Expression<Func<TEntity, bool>> where,
params Expression<Func<TEntity, object>>[] navigationProperties)
{
TEntity item = null;
IQueryable<TEntity> dbQuery = this.Context.Set<TEntity>();
//Apply eager loading
foreach (Expression<Func<TEntity, object>> navigationProperty in navigationProperties)
dbQuery = dbQuery.Include<TEntity, object>(navigationProperty);
item = dbQuery
.AsNoTracking() //Don't track any changes for the selected item
.FirstOrDefault(where); //Apply where clause
return item;
}
I would like to enhance that method to retrieve only the columns I require but still return TEntity.
I do know I have to inject a Select after the '.AsNoTracking()' but I am unsure as to how I could pass the properties in as I am only starting out with Expression Trees.
In essence I would like to be able to do this.
public class Employee
{
public int EmployeeId { get;set; }
public string EmployeeRole { get;set; }
public string EmployeeFirstName { get;set; }
public string EmployeeLastName { get;set; }
public string DOB { get;set; }
...
}
Employee employee = EmployeeRepository.Get(where: e => e.EmployeeRole == "Developer",
columns: x => x.EmployeeFirstName, x => x.EmployeeLastName,
navigationProperties: null);
Where columns is a list of expressions specifying the columns to be added to the Select clause.
Any help would be appreciated.
Thanks in advance...
Update.
I ended up with using a DTO to do the necessary querying and extraction as I couldn't find an elegant way to perform it generically. There was a solution developed by a colleague of mine but it made the repository far too complex and would have been hard to manage in the future.
So I create a StaffBasicInformation class to hold the subset of columns I use regularly. I also created an interface for it if I needed in the future. The below code sample shows the final implementation of retrieving data for the DTO.
public virtual IStaffBasicInformation GetStaffBasicInformation<TEntity2>(Expression<Func<TEntity2, bool>> where)
where TEntity2 : ActiveStaffMember
{
TEntity2 item = null;
StaffBasicInformation resultItem = null;
IQueryable<TEntity2> dbQuery = this.Context.Set<TEntity2>();
resultItem =
dbQuery.Where(where)
.Select(x => new StaffBasicInformation
{
GivenName = x.GivenName,
Department = x.Department,
Description = x.Description,
DisplayName = x.DisplayName,
Gender = x.Gender,
IID = x.IID,
Mail = x.Mail,
Title = x.Title,
ID = x.Id
})
.FirstOrDefault();
return resultItem;
}
Your return value will not be of type TEntity anymore after you have done the projection, it will be an anonymous type. You have to decide, if you want to map this anonymous type to an instance of TEntity, including mapping all navigationproperties, or return dynamic or object from your repository. Both choices are not very pleasant. Mapping would include a huge amount of reflection, which will not be very fast. By returning a dynamic type you loose all type safety. You have seen this problem allready i assume.
That said: You will need to build this expression manually. Based on this answer you can modify the
public static IQueryable SelectDynamic(this IQueryable source, IEnumerable<string> fieldNames)
to
public static IQueryable SelectDynamic(this IQueryable source, IEnumerable<Expression> fieldNames)
end extract the property names from the expressions. I would suggest to use an ExpressionVisitor for that, but you could also use the code from this answer
For the mapping you can compile the expressions and use the returned Func to retrieve the value from the anonymous type. After that you would need to use expression and find the hosting type for the selected property by using an ExpressionVisitor again. Then you will need to create a object of type of TEntity via Activator.CreateType(), and objects for every hosting type. Assign the value from Func(AnonymousType) to the created object of the hosting type using the property name from the expression. After that you have to determin the relationship between TEntity and the hosting type and build it up.
I will try to post some code tomorrow for this scenario, although i am quite sure there is a better and faster way.
Im using Entity Framework 4, I have an entity
public partial class UserEntity : EntityObject
{
public string id_user;
public string pass;
public DateTime date_added;
}
and a data transfer object with custom attributes
[DataTransferObject("UserEntity")]
public class User
{
[EntityColumn("id_user")]
public string idUser { get; set; }
[EntityColumn("pass")]
public string password { get; set; }
[EntityColumn("date_added")]
public string dateAdded { get; set; }
}
I convert the entity to dto and the dto to entity using these attributes with reflection, I'm using a generic repository for data access methods, when I want to make a query, using Enumerable.Where, to the ObjectSet of the entity, this require a Func<dynamic, bool> (dynamic because the program doesn't know the entity implementing the repository), this works but I want a Func<DTObject, bool> as parameter so I don't have to use entities in business layer, is there a way to achieve something like this?
public static Func<dynamic, bool> ConvertFunc<DTObject>(Func<DTObject, bool> predicate)
{
/* do some magic to convert func */
return newFunc;
}
Func<dynamic, bool> newFunc = ConvertFunc<User>(u => u.idUser == "admin" && u.password == "abc123");
/* newFunc = u => u.id_user == "admin" && u.pass == "abc123" */
I need to stop you right here. You are completely wrong on your approach. You need to use Queryable.Where<T>(this IQueryable<T> query, Expression<Func<T, bool>>) for EntityFramework to work. The method you would actually be using is Enumerable.Where<T>(this IEnumerable<T> set, Func<T, bool> predicate), which would do the filtering IN MEMORY. This means the road you are going down will download your entire table into memory.
For this to work you need to be playing with the System.Linq.Expressions namespace. However, if this is going to work, without some insane amount of logic (more than already exists with Expressions). Your Dto signature will be exactly the same as your EntityObject.
Given this is true, what value does your DTO add (given the maintenance cost of the class)?
The only value I see is the decoupling of your domain logic from your data storage (ie you don't have EntityObject in your domain layer). However that problem was solved years ago with POCOs.
TLDR
XY problem. Use POCO instead.
I would like to pass an IQueryable and an array of ids to a method which filters the IQueryable based on those ids.
As the ids can be either long's or int's it should be solved generically.
I came up with the following:
public static IEnumerable<T> GetModified<TId, T>(IQueryable<T> objects, TId[] ids) where T : class
{
return objects.Where(j => ids.Contains((TId)j.GetType().GetProperty("Id").GetValue(j)));
}
Unfortunately I'm getting the exception:
LINQ to Entities does not recognize the method 'System.Object GetValue(System.Object)' method, and this method cannot be translated into a store expression.
The exception is normal, as getting properties through reflection is something that clearly cannot be translated to SQL.
One thing I would try is to create a generic interface that exposes an Id property of a given type:
public interface HasId<T> {
T Id { get; set; }
}
Now you could declare your entity as implementing HasId<int>, for example, if the Id was of type int.
The next step is to modify your method like so:
public static IEnumerable<T> GetModified<TId, T>
(IQueryable<T> objects, TId[] ids) where T : class, HasId<TId>
{
return objects.Where(j => ids.Contains(j.Id));
}
Note the added generic restriction: where T : class, HasId<TId>. This enables you to write the simplified j.Id, which returns a TId value, instead of resorting to reflection.
Please note that I haven't run or tested this code; it's just an idea that I got when I saw your problem and I hope it helps.
Update:
Here's another possible solution that doesn't require that you declare interfaces or change your classes in any way:
public static IEnumerable<T> GetModified<TId, T>
(IQueryable<T> objects, TId[] ids, Expression<Func<T, TId>> idSelector)
where T : class
{
return objects.Where(j => ids.Contains(idSelector(j)));
}
What I've done here is add the Expression<Func<T, TId>> idSelector parameter, an expression that can return the Id of a given instance of T.
You would call the method like that:
var modified = GetModified(dbObjects, yourIdArray, entity => entity.Id);
(only the third parameter being new; keep the others as you have them now).
Again, I haven't tested if this works or even compiles, as I don't have a computer with VS here :(.
Entity Framework doesn't support some of the .NET methods such as GetValue() since it does not translate to SQL (which is the code actually executed to the IQueryable. Try calling ToList to get the CLR object before doing reflection:
public static IEnumerable<T> GetModified<TId, T>(IQueryable<T> objects, TId[] ids) where T : class
{
return objects.ToList().Where(j => ids.Contains((TId)j.GetType().GetProperty("Id").GetValue(j)));
}