Unable to translate the LINQ expression while using Group by - linq

Question

I have an Asp.net core web API that uses EF Core DB first approach. I have the following Tables in my SQL server database.
Application table - contains a list of Applications.
Role table - contains a list of Roles (user, admin, Super Admin, etc.)
User table - contains a list of Users
User Role table - contains a mapping between User and Role table
Feature tables - contains a list of Features (Home page, user management, etc.)
RoleFeature table - contains the mapping between Feature and Role table
I am trying to get the RoleName and list of features for the given UserId and appId.
Below is the Linq query I have so far:
RoleDto role =
from a in ctx.Application.Where(x => x.ApplicationId == appId)
from r in ctx.Role.Where(x => x.ApplicationId == a.ApplicationId)
from ur in ctx.UserRole.Where(x => x.UserId == userId && x.RoleId == r.RoleId)
from rf in ctx.RoleFeature.Where(x => x.RoleId == ur.RoleId)
from f in ctx.Feature.Where(x => x.FeatureId == rf.FeatureId).Where(x => x.IsActive)
group new { r.RoleName, f.FeatureId } by ur.RoleId into g
select new RoleDto
{
Name = g.Select(x => x.RoleName).FirstOrDefault(),
FeatureIds = g.Select(x => x.FeatureId).ToList()
}.AsNoTracking()
However, I am getting an error saying unable to translate the LINQ expression.

Answer 1

The problem is with the FirstOrDefault() in the select section of the "groupby" query.
You are probably using ef core version higher than 2.1 and there are some changes in handling groupby query. You should take a look at the following link :
https://learn.microsoft.com/en-us/ef/core/what-is-new/ef-core-2.1#linq-groupby-translation
Before version 2.1, in EF Core the GroupBy LINQ operator would always
be evaluated in memory. We now support translating it to the SQL GROUP
BY clause in most common cases.
So the query should be translated to SQL GROUP BY, but methods such as FirstOrDefault() can not be translated. As a fast solution you can change FirstOrDefault() with Max(), Min() or other aggregate functions that are supported in DB.

Answer 2

You don't need to select the Application. You can start by selecting Role directly as -
from r in dbCtx.Role.Where(x => x.ApplicationId == appId)
This will simplify the final SQL generated by EF. So the query will be faster.
If a User has multiple Role then you are trying to take the first one. You should do that when you are selecting Role as -
from r in ctx.Role.Where(x => x.ApplicationId == a.ApplicationId).Take(1)
Finally, you can fetch a list of RoleName and FeatureId, and then do the grouping on client side -
var query =
from r in dbCtx.Role.Where(x => x.ApplicationId == appId).Take(1)
from ur in dbCtx.UserRole.Where(x => x.UserId == userId && x.RoleId == r.RoleId)
from rf in dbCtx.RoleFeature.Where(x => x.RoleId == ur.RoleId && x.Feature.IsActive)
select new
{
RoleName = rf.Role.RoleName,
FeatureId = rf.FeatureId
};
var roleDto = query.AsNoTracking()
.AsEnumerable()
.GroupBy(p => p.RoleName)
.Select(g => new RoleDto
{
Name = g.Key,
FeatureIds = g.Select(p => p.FeatureId).ToList()
})
.FirstOrDefault();