Question

Situation:

TableParent with 2 primaryKeys, ParentKey1 and ParentKey2
TableChild with 1 primaryKey, ChildKey
TableConnector with columns ParentKey1, ParentKey2 and ChildKey

This is where I think I should go with my Linq query. Notice I’m fetching all childs belonging to a parent so I have it’s keys as parameters.

var query = from conn in db.TableConnector
            join child in db.TableChild on conn.ChildKey equals child.childKey
            join par in db.TableParent on conn.ParentKey1 equals par.parentkey1 into connGroup
            from co in connGroup
            where co.ParentKey1 == Parameter1
            Select child;

Well I tthink this works up to a point, let’s say if parent had only one key, am I right?

I guess I have to join some more into a second group but I’m currently lost.

Answer 1

Firstly, you don’t have two primary keys on your table. You can only have one primary key (hence the name primary). It sounds like you have a composite primary key, which means a key that is composed of more than one column.

I’m not sure what problem you are trying to solve, but it seems to be retrieving all TableChild rows for a given TableParent key. It should be something like this:

db.TableParent
    .Single(parent => parent.ParentKey1 == key1 && parent.ParentKey2 == key2)
    .TableConnectors.Select(connector => connector.TableChild)

If you have your tables mapped correctly on your Linq-to-Sql designer then you don’t have to manually join them – that’s what the Linq-to-Sql code generation does for you.

For example, when you have a TableConnector you will be able to retrieve the TableChild rows for it using something like this

TableConnector t = db.TableConnectors.First();
List<TableChilds> tableChilds = tableConnector.TableChilds.ToList();