GroupJoin: exception thrown: System.InvalidOperationException - entity-framework

Question

I'm trying to write a query to get all restaurant tables if exists or not a opened sale on it.
if a sale exists on a table I want to get the sum and couple details.that is my code:
db.SALETABLES
.GroupJoin(
db.SALES.Where(c => c.CLOSEDTIME == null),
t => t.ID,
sa => sa.ID_TABLE,
(ta, s) => new
{
ta.ID,
ta.DESCRIPTION,
NR_SALE = s.Any() ? s.First().NR_SALE : 0,
IDSALE = s.Any() ? s.First().ID : 0,
IDUSER = s.Any() ? s.First().IDUSER : 0,
USERNAME = s.Any() ? s.First().USERS.USERNAME :"" ,
SALESUM = s.Any() ? s.First().SALES_DETAIL.Sum(p => p.PRICE * p.CANT) : 0
}
but got this error:
Exception thrown: 'System.InvalidOperationException' in
System.Private.CoreLib.dll
thanks for any help

Answer 1

You don't specify the exception, but I assume it's about client-side evaluation (CSE), and you configured EF to throw an exception when it occurs.
It may be First() that triggers CSE, or GroupJoin. The former can easily be fixed by using FirstOrDefault(). The GroupJoin has more to it.
In many cases it isn't necessary to use GroupJoin at all, of Join, for that matter. Usually, manually coded joins can and should be replaced by navigation properties. That doesn't only make the code better readable, but also avoids a couple of issues EF 2.x has with GroupJoin.
Your SaleTable class (I'm not gonna follow your database-driven names) should have a property Sales:
public ICollection<Sale> Sales { get; set; }
And if you like, Sale could have the inverse navigation property:
public SaleTable SaleTable { get; set; }
Configured as
modelBuilder.Entity<SaleTable>()
.HasMany(e => e.Sales)
.WithOne(e => e.SaleTable)
.HasForeignKey(e => e.SaleTableId) // map this to ID_TABLE
.IsRequired();
Now using a table's Sales property will have the same effect as GroupJoin —a unique key, here a SaleTable, with an owned collection— but without the issues.
The next improvement is to simplify the query. In two ways. 1. You repeatedly access the first Sale, so use the let statement. 2. The query is translated into SQL, so don't worry about null references, but do prepare for null values. The improved query will clarify what I mean.
var query = from st in db.SaleTables
let firstSale = st.Sales.FirstOrDefault()
select new
{
st.ID,
NrSale = (int?)firstSale.NrSale ?? 0,
IdSale = (int?)firstSale.ID ?? 0,
...
SalesSum = (int?)firstSale.SalesDetails.Sum(p => p.Price * p.Cant) ?? 0
}
Using NrSale = firstSale.NrSale, would throw an exception for SaleTables without Sales (Nullable object must have a value).

Answer 2

Since the exception is by the EF Core infrastructure, apparently you are hitting current EF Core implementation bug.
But you can help EF Core query translator (thus avoiding their bugs caused by missing use cases) by following some rules when writing your LINQ to Entities queries. These rules will also eliminate in most of the cases the client evaluation of the query (or exception in EF Core 3.0+).
One of the rules which is the origin of issues with this specific query is - never use First. The LINQ to Objects behavior of First is to throw exception if the set is empty. This is not natural for SQL which naturally supports and returns NULL even for values which normally do not allow NULL. In order to emulate the LINQ to Objects behavior, EF Core has to evaluate First() client side, which is not good even if it works. Instead, use FirstOrDefault() which has the same semantics as SQL, hence is translated.
To recap, use FirstOrDefault() when you need the result to be a single "object" or null, or Take(1) when you want the result to be a set with 0 or one elements.
In this particular case, it's better to incorporate the 0 or 1 related SALE rule directly into the join subquery, by removing the GroupJoin and replacing it with SelectMany with correlated Where. And the Any() checks are replaced with != null checks.
With that said, the modified working and fully server translated query looks like this:
var query = db.SALETABLES
.SelectMany(ta => db.SALES
.Where(s => ta.ID == s.ID_TABLE && s.CLOSEDTIME == null).Take(1), // <--
(ta, s) => new
{
ta.ID,
ta.DESCRIPTION,
NR_SALE = s != null ? s.NR_SALE : 0,
IDSALE = s != null ? s.ID : 0,
IDUSER = s != null ? s.IDUSER : 0,
USERNAME = s != null ? s.USERS.USERNAME : "",
SALESUM = s != null ? s.SALES_DETAIL.Sum(p => p.PRICE * p.CANT) : 0
});