Linq返回结果作为另一个linq查询的条件

Question

var query_1 = from m in db.tableA
                        join n in db.tableB
                        on m.OwnerID equals n.OwnerID into tabA
                        from a in tabA
                        join o in db.tableC
                        on m.OwnerID equals o.OwnerID 
                        where m.UserID == userSessionID
                        && m.ActiveStatus == 1
                        select new { OwnerName = m.OwnerName };

var query_2 = from m in db.tableD
                        join n in db.tableE
                        on m.OwnerID equals n.OwnerID into tabX
                        from a in tabX
                        join o in db.tableF
                        on m.OwnerID equals o.OwnerID 
                        where {?????????????}
                        select new { ...... };

From the above code, query_1 will return me a list of elements retrieve from database. What I need is I only want the first item in the query_1 and using this element's column as a condition in my query_2. All this while i was using foreach to loop thru query_1 to get all the element but this time i only want the first item. Note that the list of query_1 may return me null and hence hardcode is not allow. Any luck?

Answer 1

after query_1;

var firstOwner = query_1.FirstOrDefault() ?? string.Empty;

then in query_2, assuming m ( db.tableD ) has an OwnerName property...

where m.OwnerName = firstOwner;

Version 2 , If you need an OwnerId from query_1 :

change it to

select new { OwnerName = m.OwnerName, OwnerId = m.OwnerId };

than

var firstOwner = query_1.FirstOrDefault();
var firstOwnerId = firstOwner == null ? -1 : firstOwner.OwnerId;

and in query_2

where m.OwnerId = firstOwnerId;

Answer 2

you can use .FirstOrDefault() to get the first element from first query then you can use its value as needed.

You may also like to have a look at and this

Answer 3

Try putting it on a list then get the top most record as .First()?

        List<object> first = new List<object>();
        first = query_1.toList();

then on your query_2 put this:

        where m.OwnerId = first.First();

EDITED:

Or simply put this on your query_2:

        where m.OwnerId = query_1.First();