如何使用LINQ扩展方法创建多个联接？

Question

I'm having trouble using LINQ method calls with multiple joins. I'm trying to do something like this:

            if (!isDepSelect)
            {                
                query = (from Items in db.DEPARTMENTs
                             select Items);
            }
            else
            {                    
                    query = (from Items in db.DEPARTMENTs
                             from gDept in db.DEPT_PROFILE
                             from wAccess in db.WEB_ACCESS
                             where Items.DEPT_CODE == gDept.DEPT_CODE && gDept.USER_ID == wAccess.USER_ID && wAccess.EMP_ID == id
                             select Items);
            }

I had done this:

IQueryable<DEPARTMENT> query = db.DEPARTMENTs;


            if (isDepSelect)
            { 
                query = query.Join(db.DEPT_PROFILE,depts => depts.DEPT_CODE,prof => prof.DEPT_CODE,(depts, prof) => depts);    
            }

But now I don't know how to add the JOIN of DEPT_PROFILE table with the WEB_ACCESS table and the condition of the EMP_ID = id.

The reason I'm doing this is that the isDepSelect boolean is not the only condition that this query will change its relations and I need someway to add this relations without repeating my LINQ for each of my conditions.

Thank you for your time.

Answer 1

Try with,

List<DEPARTMENTs> list =   db.DEPARTMENTs.Join(db.DEPT_PROFILE, dept => dept.DEPT_CODE, prof => prof.DEPT_CODE, (dept,prof) => new {dept, prof})
            .Join(Wdb.WEB_ACCESS, depts => depts.prof.USER_ID,web => web.USER_ID,(depts,web) => new { depts, web})
            .Where(result => result.web.EMP_ID== id).Select(s => s.depts.dept).ToList<DEPARTMENTs>();

Answer 2

If you have your associations setup, you can do this without any joins in your code at all:

query = db.DEPARTMENTs
           .Any(item => item.DEPT_PROFILEs
                .Any(gDept => gDept.WEB_ACCESSs
                     .Any(wAccess => wAccess.EMP_ID == id)));

Of course this is assuming a 1-m relationship between each of the objects in the graph. You can eliminate some of the Any methods if there are 1-0..1 relationships in the graph as necessary.

Answer 3

you should use the equals operator...

query = from Items in db.DEPARTMENTs
                             from gDept in db.DEPT_PROFILE
                             join wAccess in db.WEB_ACCESS on 
gDept.DEPT_CODE equals Items.DEPT_CODE
                             select Items;

thats just a snippet of your example query, but you can see how i am using the join operator to introduce a 2nd table and the equals operator to declare the joining columns.

Answer 4

This should work:

    query = (from Items in db.DEPARTMENTs 
             join gDept in db.DEPT_PROFILE 
                on Items.DEPT_CODE equals gDept.DEPT_CODE
             join wAccess in db.WEB_ACCESS
                on gDept.USER_ID equals wAccess.USER_ID
             where  wAccess.EMP_ID == id
             select Items);