= Why and when I would use an EntitySetExpression? =
This question is closely related with member access expressions,
specifically with EntitySets member accesses. Here some examples:
1- Employees.Select(e=> e.LastName)
2- Employees.Select(e=> e.Orders)
3- Employees.Select(e=> e.Orders.Select(o=>o))
4- Employee.Select (e=> e.Orders.Select(o=>o.OrderDetails))
IMO examples 1 and 3 are currently well managed
1- It uses ColumnExpression for translating "e.LastName"
3- It uses a TableExpression for translating "e.Orders" since it is
needed for the subquery: Select(o=>o)
But... what Does it happen with example 2 and 4? they are a mixture of
1 and 3. The key point is the return type of each query:
1- IEnumerable<string>
2- IEnumerable<EntitySet<Order>>
3- IEnumerable<Order>
4- IEnumerable<EntitySet<OrderDetail>>
That's why examples 2nd and 4th examples belongs to the same family of queries.
- 2nd and 4th examples are candidates to use lazy loading.
- 1st and 3rd examples should be continue working as now.
= example 2 vs examples 1 and 3 =
Regards the member access expression:
- It isn't like a ColumnExpression since it is a relation 1:n to other
table. However it shares a thing: In lazy loading, the rendered sql
should be something similar to a ColumnExpression, basically rendering
the foreign-key column name.
- It appear to be a Table expression, but it is different since you
don't need a join with another table (in lazy loading) .
= The current implementation =
In our current implementation, 2nd and 3rd examples, both member
access expressions are translated as a TableExpression.
Generally TableExpression is working fine with subqueries (3rd
example). However it doesn't work fine with 2nd (it crashes when the
rowObjectCreator in the QueryRunning stage is tried to be casted to
Func<IDataRecord,MappingContext,EntitySet<Order>>. A CastException is
thrown since the truth is that the function's type is
Func<IDataRecord,MappingContext,Order>. TableExpression's type blame!
= Again, example 2 vs examples 1 and 3 =
IMO To translate the member access expression in the 2nd example as a
TableExpression's type is conceptually wrong.
TableExpression's type should be an entity type (ie: Employee) but it
shouldn't be an "enumeration .of an entity type" (ie:
EntitySet<Employee>). In fact that's how it is working and that's why
it is crashing in QueryRunning stage .
Besides EntitySet member accesses (without subqueries) need particular
code in the rowObjectCreator method where to invoke the
EntitySet.SetSource method and assign the query. Wich query do we have
to assign?
As I said in the first mail, something like:
Func<.., .., EntitySet<OrderDetail>>objectRowCreator =
delegate(...,...){
....
var EmployeeID = row.GetIntegerAt(index);
var value = new EntitySet<Order>();
value.SetSource(db.Orders.Where(o=>o.EmployeeID=EmployeeID)));
...
return value;
}
= example 4 vs examples 1 and 3 =
>From my point of view 4th example is a particular case of the 2nd
example, but the final objectRowCreator should be something like this:
Func<.., .., EntitySet<*OrderDetail*>>objectRowCreator =
delegate(...,...){
....
var EmployeeID = row.GetIntegerAt(index);
var value = new EntitySet<Order>();
value.SetSource(db.Orders.Where(o=>o.EmployeeID=EmployeeID).*Orders.Select(o=>o.OrderDetails)*));
...
return value;
}
= Then why we need an EntitySetExpression ? =
1º The generated sql is like a column expression and not as a
TableExpression's generated sql.
2º We need don't destroy and to store the "c# compiler's original
subquery" in the ExpressionDispatcher.Analyze stage, and use it after
in the CompileRowCreator stage, when the rowObjectCreator is being
created as we have already explained above.
[In 4th example we should store this expression:
Orders.Where(o=>o.EmployeeID=EmployeeID).Orders.Select(o=>o.OrderDetails)
]
Hope it helps.
Nonetheless I'm not sure if I have explained the problem correctly.
Regards.
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