Question

Consider this code:

var query = from groupRole in CurrentItem.MEMGroupRoles 
            select groupRole.MEMRole;
this.AvailableRoles = this.allRoles.Except(query.AsEnumerable()).ToList();

In this code I take allRoles except those roles that CurrentItem already have. 2 issues:

1. It doesn’t work because I compare on objects and those objects are different instances
2. I don’t like 2 lines and like to improve.

Here is pseudo-code on what I really need to do right now:

var queryIds = from groupRole in CurrentItem.MEMGroupRoles 
               select groupRole.MEMRole.RoleId;
this.AvailableRoles = this.allRoles.Except(where RoleId query.AsEnumerable()).ToList();

How do I write query like this?

EDIT:

explanation:

1. allRoles contains list of MEMRole objects
2. CurrentItem.MEMGroupRoles contains list of MEMGroupRole objects and each MEMGroupRole contains MEMRole inside

I want to SELECT all MEMRole objects that’s inside allRoles EXCEPT those MEMRoles that burries inside CurrentItem. First code snippet would work, but I need to compare MEMRole to MEMRole by MEMRole.RoleId since it’s a different instances of the same database entity.

Answer 1

Following the approach you suggested:

var ids = CurrentItem.MEMGroupRoles.Select(g => g.MMERole.RoleId);
this.AvailableRoles = this.allRoles.Where(r => ids.All(i => i != r.RoleId));

Alternatively (althought I wouldn’t go that road), if you must have single query, you can append both roles collections (current and all), group them by RoleId and pick groups that only have single member:

this.AvailableRoles = CurrentItem.MEMGroupRoles
    .Select(g => g.MEMRole)
    .Concat(this.allRoles)
    .GroupBy(r => r.RoleId)
    .Where(g => g.Count() == 1)
    .Select(g => g.First());

This results in roles that weren’t in CurrentItem.MEMGroupRoles collection. But once again, it’s just … for sport 🙂