Entity Framework LINQ Expression on object within object

Question

Editing this question in the hope to make it clearer.

We have entity framework code first setup. I've simplified two classes for the purposes of example, in reality there are around 10+ more classes similar to the 'Record', where Item is a navigational property/foreign key.

Item class:

public class Item
{
    public int Id { get; set; }
    public int AccountId { get; set; }
    public List<UserItemMapping> UserItemMappings { get; set; }
    public List<GroupItemMapping> GroupItemMappings { get; set; }
}

Record class:

public class Record 
{
    public int ItemId { get; set; }
    public Item Item { get; set; }
}

this.User is an injected user object into each repo and is contained on the repository base. We have an Item repository with the following code:

var items = this.GetAll()
    .Where(i => i.AccountId == this.User.AccountId);

I created the follow expression on the repository base to easily filter on that (in the hope of re-use). We cannot use static extension methods due to how LINQ to entities works (System.NotSupportedException "LINQ to Entities does not recognize the method X and this method cannot be translated into a store expression.").

protected Expression<Func<Item, bool>> ItemIsOnAccount()
{
    return item => item.AccountId == this.User.AccountId;
}

I have solved the case of the above, by doing this:

var items = this.GetAll().Where(this.ItemIsOnAccount());

We have additional filtering based on user permissions within that account (again, another case where I do not want to repeat this code in every repo we have):

protected Expression<Func<Item, bool>> SubUserCanAccessItem()
{
    return item => this.User.AllowAllItems 
        || item.UserItemMappings.Any(d => d.UserId.Value == this.User.Id) 
        || item.GroupItemMappings.Any(vm => 
            vm.Group.GroupUserMappings
                .Any(um => um.UserId == this.User.Id));
}

Which I am able to use as follows:

    var items = this.GetAll().Where(this.SubUserCanAccessItem());

However, what we also need, in the Record repository is a way to solve the following:

var records = this.GetAll()
    .Where(i => i.Item.AccountId == this.User.AccountId);

Because Item is a single navigational property, I do not know how to apply the expressions I have created to this object.

I want to reuse the expression I created in the repo base on all of these other repos, so that my 'permission based' code is all in the same place, but I cannot simply throw it in because the Where clause in this case is of Expression< Func < Record,bool >>.

Creating an interface with a method of:

Item GetItem();

on it and putting it on the Record class does not work because of LINQ to entities.

I cannot also create a base abstract class and inherit from it, because there could be other objects than Item that need to be filtered on. For instance a Record could also have a 'Thing' on it that has permission logic. Not all objects will require to be filtered by 'Item' and 'Thing', some by only one, some by another, some by both:

var items = this.GetAll()
    .Where(this.ItemIsOnAccount())
    .Where(this.ThingIsOnAccount());

var itemType2s = this.GetAll().Where(this.ThingIsOnAccount());

var itemType3s = this.GetAll().Where(this.ItemIsOnAccount());

Due to this having a single parent class would not work.

Is there a way in which I can reuse the expressions I have already created, or at least create an expression/modify the originals to work across the board within the OTHER repos that of course return their own objects in a GetAll, but all have a navigation property to Item? How would I need to modify the other repos to work with these?

Thanks

Answer 1

The first step for expression reusability is to move the expressions to a common static class. Since in your case they are tied to User, I would make them User extension methods (but note that they will return expressions):

public static partial class UserFilters
{
    public static Expression<Func<Item, bool>> OwnsItem(this User user)
        => item => item.AccountId == user.AccountId;

    public static Expression<Func<Item, bool>> CanAccessItem(this User user)
    {
        if (user.AllowAllItems) return item => true;
        return item => item.UserItemMappings.Any(d => d.UserId.Value == user.Id) ||
            item.GroupItemMappings.Any(vm => vm.Group.GroupUserMappings.Any(um => um.UserId == user.Id));
    }
}

Now the Item repository would use

var items = this.GetAll().Where(this.User.OwnsItem());

or

var items = this.GetAll().Where(this.User.CanAccessItem());

In order to be reusable for entities having Item reference, you would need a small helper utility for composing lambda expressions from other lambda expressions, similar to Convert Linq expression "obj => obj.Prop" into "parent => parent.obj.Prop".

It's possible to implement it with Expression.Invoke, but since not all query providers support for invocation expressions (EF6 doesn't for sure, EF Core does), as usual we'll use a custom expression visitor for replacing a lambda parameter expression with another arbitrary expression:

public static partial class ExpressionUtils
{
    public static Expression ReplaceParameter(this Expression expression, ParameterExpression source, Expression target)
        => new ParameterReplacer { Source = source, Target = target }.Visit(expression);

    class ParameterReplacer : ExpressionVisitor
    {
        public ParameterExpression Source;
        public Expression Target;
        protected override Expression VisitParameter(ParameterExpression node)
            => node == Source ? Target : node;
    }
}

And the two composing functions are as follows (I don't like the name Compose, so sometimes I use the name Map, sometimes Select, Bind, Transform etc., but functionally they do the same. In this case I'm using Apply and ApplyTo, with the only difference being the transformation direction):

public static partial class ExpressionUtils
{
    public static Expression<Func<TOuter, TResult>> Apply<TOuter, TInner, TResult>(this Expression<Func<TOuter, TInner>> outer, Expression<Func<TInner, TResult>> inner)
        => Expression.Lambda<Func<TOuter, TResult>>(inner.Body.ReplaceParameter(inner.Parameters[0], outer.Body), outer.Parameters);

    public static Expression<Func<TOuter, TResult>> ApplyTo<TOuter, TInner, TResult>(this Expression<Func<TInner, TResult>> inner, Expression<Func<TOuter, TInner>> outer)
        => outer.Apply(inner);
}

(Nothing special there, code provided for completeness)

Now you could reuse the original filters by "applying" them to a expression which selects Item property from another entity:

public static partial class UserFilters
{
    public static Expression<Func<T, bool>> Owns<T>(this User user, Expression<Func<T, Item>> item)
        => user.OwnsItem().ApplyTo(item);

    public static Expression<Func<T, bool>> CanAccess<T>(this User user, Expression<Func<T, Item>> item)
        => user.CanAccessItem().ApplyTo(item);
}

and add the following to the entity repository (in this case, Record repository):

static Expression<Func<Record, Item>> RecordItem => entity => entity.Item;

which would allow you to use there

var records = this.GetAll().Where(this.User.Owns(RecordItem));

or

var records = this.GetAll().Where(this.User.CanAccess(RecordItem));

This should be enough to satisfy your requirements.

---

You can go further and define an interface like this

public interface IHasItem
{
    Item Item { get; set; }
}

and let the entities implement it

public class Record : IHasItem // <--
{
    // Same as in the example - IHasItem.Item is auto implemented
    // ... 
}

then add additional helpers like this

public static partial class UserFilters
{
    public static Expression<Func<T, Item>> GetItem<T>() where T : class, IHasItem
        => entity => entity.Item;

    public static Expression<Func<T, bool>> OwnsItem<T>(this User user) where T : class, IHasItem
        => user.Owns(GetItem<T>());

    public static Expression<Func<T, bool>> CanAccessItem<T>(this User user) where T : class, IHasItem
        => user.CanAccess(GetItem<T>());
}

which would allow you omit the RecordItem expression in the repository and use this instead

var records = this.GetAll().Where(this.User.OwnsItem<Record>());

or

var records = this.GetAll().Where(this.User.CanAccessItem<Record>());

Not sure if it gives you a better readability, but is an option, and syntactically is closer to Item methods.

For Thing etc. just add similar UserFilters methods.

---

As a bonus, you can go even further and add the usual PredicateBuilder methods And and Or

public static partial class ExpressionUtils
{
    public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> left, Expression<Func<T, bool>> right)
        => Expression.Lambda<Func<T, bool>>(Expression.AndAlso(left.Body,
            right.Body.ReplaceParameter(right.Parameters[0], left.Parameters[0])), left.Parameters);

    public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> left, Expression<Func<T, bool>> right)
        => Expression.Lambda<Func<T, bool>>(Expression.OrElse(left.Body,
            right.Body.ReplaceParameter(right.Parameters[0], left.Parameters[0])), left.Parameters);
}

so you could use something like this if needed

var items = this.GetAll().Where(this.User.OwnsItem().Or(this.User.CanAccessItem()));

in the Item repository, or

var records = this.GetAll().Where(this.User.OwnsItem<Record>().Or(this.User.CanAccessItem<Record>()));

in the Record repository.

Answer 2

I can't really tell if this could work in your case, depends on how your entities might be setup, but one thing you can try is to have an interface like IHasItemProperty with a GetItem() method and have the entities where you want to use this implement that interface. Something like this :

public interface IHasItemProperty {
    Item GetItem();
}

public class Item: IHasItemProperty {

    public Item GetItem() {
       return this;
    }

    public int UserId {get; set;}
}

public class Record: IHasItemProperty {
    public Item item{get;set;}

    public Item GetItem() {
        return this.item;
    }
}

public class Repo
{
    protected Expression<Func<T, bool>> ItemIsOnAccount<T>() where T: IHasItemProperty
    {
        return entity => entity.GetItem().UserId == 5;
    }

}

I have used an int just to make things simpler.