Web API, OData, EF5, Union: The 'Distinct' operation cannot be applied to the collection ResultType of the specified argument

Question

I'm working on a Web API prototype using the OData Nuget package. I'm having some issues getting a LINQ to EF query to work. Here are my data model. It has been highly simplified. I'm trying to get the query to work using this DTO: ``` public class Product { public int Id { get; set; } public string Name { get; set; } public IEnumerable Orders { get; set; } } public class Order { public int Id { get; set; } public DateTime Date { get; set; } } ``` The query looks like this: ``` [Queryable] public IQueryable Get() { var productA = _context.ProductA .Select(p => new Product { Id = p.id, Name = p.name, Orders = p.ProductAOrders.Select(o => new Order { Id = o.OrderId, Date = o.Orders.Date, }) }); var productB = _context.ProductB .Select(p => new Product { Id = p.Id, Name = p.Name, Orders = p.ProductBOrders.Select(o => new Order { Id = o.OrderId, Date = o.Orders.Date, }) }); return productA.Union(productB); } ``` When trying to Union the two queries I get this error: ```

Accepted Answer

Looks like an EF bug. I assume you are trying to get MEST (multiple entity sets of same type) working. Instead of the query that you had, you can try,

```
public IQueryable<Product> Get()
{
    var productA = _context.ProductA
                            .Select(p => new Product
                            {
                                Id = p.id,
                                Name = p.name,
                            });

var productB = _context.ProductB
                            .Select(p => new Product
                            {
                                Id = p.Id,
                                Name = p.Name,
                            });

return
        productA
        .Union(productB)
        .Select(p => new Product
        {
            Id = p.Id,
            Name = p.Name,
            Orders = _context.Orders
                    .Where(o => o.ProductA.Id == p.Id || o.ProductB.Id == p.Id)
                    .Select(o => new Order
                    {
                        Id = o.OrderId,
                        Date = o.Orders.Date,
                    })
        });
}
```

The idea is to take the navigation properties out of the union and add them later back. This would work only if Orders had back pointer to ProductA or ProductB.

Web API, OData, EF5, Union: The 'Distinct' operation cannot be applied to the collection ResultType of the specified argument

Solution

Problem