连接Linq查询内的Linq查询结果

Question

The issue that I have is, the query works okay apart from problem with the part of the query which does the concatenation, the concatenation query works outside the query but when I put inside it will not work and get the error at the end under the code.

var list = (from x in context.Contacts
where !x.DeleteFlag && !x.EmptyFlag
select new models.Contacts.list
{

// CONTACT
Contact = x,

// CONTACT'S PHONE
Phone =
   context.EContacts.Where(e => e.id == x.PrimaryPhoneid)
  .Select(e => e.Title).FirstOrDefault(),

// CONTACT'S EMAIL
Email =
   context.EContacts.Where(e => e.id == x.PrimaryEmailid)
   .Select(e => e.Title).FirstOrDefault(),

// CONTACT'S ACCOUNT
Account =
   context.Accounts.Where(e => e.id == x.Parentid)
   .Select(e => e.AccountName).FirstOrDefault(),

// Problem Is Here With This Query
tag =  string.Concat((from HE in context.HashTagEntities
   join t in context.Accounts on HE.ParentEntityid equals t.id
   where HE.ParentEntityId == 3 &&
   t.AccountName == context.Accounts.Where(e => e.id == x.Parentid).Select(e => e.AccountName)
   .FirstOrDefault()
   from tag in context.HashTags
   where HE.HashTagid == tag.id
   select tag.HashTagText).ToArray()),

}).OrderBy(o => o.Contact.FirstName);

error:

{"LINQ to Entities does not recognize the method 'System.String Concat(System.String[])' method, and this method cannot be translated into a store expression."} System.SystemException {System.NotSupportedException}

Answer 1

The problem is exactly what the error is telling you - String.Concat cannot be translated into a SQL query. So break the query into two parts - one that queries the database and the second that brings the data into memory before doing the concat.

    var listQuery =
        from x in context.Contacts
        where !x.DeleteFlag && !x.EmptyFlag
        orderby x.FirstName
        select new
        {
            Contact = x,
            Phone =
                context.EContacts.Where(e => e.id == x.PrimaryPhoneid)
                .Select(e => e.Title).FirstOrDefault(),
            Email =
                context.EContacts.Where(e => e.id == x.PrimaryEmailid)
                .Select(e => e.Title).FirstOrDefault(),
            Account =
                context.Accounts.Where(e => e.id == x.Parentid)
                .Select(e => e.AccountName).FirstOrDefault(),
            tags =
                from HE in context.HashTagEntities
                join t in context.Accounts on HE.ParentEntityid equals t.id
                where HE.ParentEntityId == 3 &&
                    t.AccountName == context.Accounts.Where(e => e.id == x.Parentid).Select(e => e.AccountName)
                    .FirstOrDefault()
                from tag in context.HashTags
                where HE.HashTagid == tag.id
                select tag.HashTagText,                
        };

    var list =
        from x in listQuery.ToArray()
        select new models.Contacts.list()
        {
            Contact = x.Contact,
            Phone = x.Phone,
            Email = x.Email,
            Account = x.Account,
            tags = String.Concat(x.tags.ToArray()),
        };

Answer 2

Why do you use string.Contat ? Just do summ.

tag = mainQuery.AccountName + (childQuery.ToArray().Aggregate(x,y)=> x + "," + y)

If method above didn't help I suggest you to make two separate queries. First will fetch contacts and the second will compute HashTags. Then just merge it. It will improve performance.

And the best way in my opinion is to use native T-SQL with entity framework.

class MyDataRow
{
    public int Id {get;set;}
    public double FieldB {get;set;}
    public string FieldC {get;set;}
}

string queryText = @"
SELECT 
    t1.Id, 
    t1.FieldB, 
    (SELECT hashTag FROM table_2 WHERE contactId = t1.Id) AS FieldC
FROM table_1 AS t1
";

// EF will map properties automatically and you do not have to write and configure stored procedure...
List<MyDataRow> rows = context.ExecuteStoreQuery<MyDataRow>(queryText).ToList();