Question

I’m struggling with an exception using linq-to-sql Concat()

I’ve got 2 tables.
The first table, ParsedMessages, has the following fields

* ParsedMessageID (int)
* MessageTypeID (int)
* TextMessage (varchar(max)) 

The second table, ParsedMessageLinks, has the following fields

* ParsedMessageID (int)
* AnotherID (int)
* NumberOfOccurences (int)

This is what I need to achieve using a single linq query but I’m not sure if it’s possible or not.

• Through a join, retrieves ParsedMessage records that links to a certain AnotherID. In example SQL and linq code, the AnotherID will have the value 0 just for the purpose of having an example.

• For each ParsedMessage record, I also need the NumberOfOccurences (field of table #2)

• Retrieve only the top(100) ParsedMessage records for each MessageTypeID. So for example, if there is 275 records in ParsedMessages that links to AnotherID==0 where the first 150 records have MessageTypeID == 0 and the remaining 125 records having MessageTypeID == 1, I want my query to end up returning 200 records, the top(100) with MessageTypeID == 0 and the top(100) with MessageTypeID == 1

After a lot of search, I’ve found that the plain SQL equivalent of I what I want to do is this. I knew that this exists first end, but I tried to find something else without Union all at first and fail to do so (my SQL knowledge is not that good) :

SELECT TOP(100) PM.*,
PML.NumberOfOccurences FROM
ParsedMessages PM INNER JOIN
ParsedMessageLinks PML ON
PM.ParsedMessageID =
PML.ParsedMessageID WHERE
PML.AnotherID = 0 AND PM.MessageTypeID
= 0 ORDER BY PM.ParsedMessageID DESC UNION ALL

SELECT TOP(100) PM.*,
PML.NumberOfOccurences FROM
ParsedMessages PM INNER JOIN
ParsedMessageLinks PML ON
PM.ParsedMessageID =
PML.ParsedMessageID WHERE
PML.AnotherID = 0 AND PM.MessageTypeID
= 1 ORDER BY PM.ParsedMessageID DESC UNION ALL

SELECT TOP(100) PM.*,
PML.NumberOfOccurences FROM
ParsedMessages PM INNER JOIN
ParsedMessageLinks PML ON
PM.ParsedMessageID =
PML.ParsedMessageID WHERE
PML.AnotherID = 0 AND PM.MessageTypeID
= 2 ORDER BY PM.ParsedMessageID DESC

So basically, the only way to retrieve the data I need is to do 3 sql queries in a single pass where only the PM.MessageTypeID is different for each query.

Now I wanted to achieve this using linq-to-sql. After googling, I’ve found that I could use the Linq Concat() method to reproduce a SQL Union All.
Here are some links pointing to what I thought would work :
http://blog.benhall.me.uk/2007/08/linq-to-sql-difference-between-concat.html

EF. How to union tables, sort rows, and get top entities?

I end up having this exception :
System.IndexOutOfRangeException : “Index was outside the bounds of the array.”

Here’s the faulty code :

IQueryable<MyObject> concatquery;
int[] allMessageTypeIDs = new int[] { 0, 1, 2 };
for (int mt = 0; mt < allMessageTypeIDs.Length; mt++)
{
    if (mt == 0)
    {
        concatquery = (from pm in db.ParsedMessages
                       join pml in db.ParsedMessageLinks on pm.ParsedMessageID equals pml.ParsedMessageID
                       where pml.AnotherID == 0 && pm.MessageTypeID == allMessageTypeIDs[mt]
                       orderby pm.ParsedMessageID descending
                       select new MyObject
                       {
                           NumberOfOccurences = pml.Occurrences,
                           ParsedMessage = pm
                       }).Take(100);
    }
    else
    {
        concatquery = concatquery.Concat(from pm in db.ParsedMessages
                                        join pml in db.ParsedMessageLinks on pm.ParsedMessageID equals pml.ParsedMessageID
                                        where pml.AnotherID == 0 && pm.MessageTypeID == allMessageTypeIDs[mt]
                                        orderby pm.ParsedMessageID descending
                                        select new MyObject
                                        {
                                            NumberOfOccurences = pml.Occurrences,
                                            ParsedMessage = pm
                                        }).Take(100);
    }
}
var results = concatquery.ToArray();

I’ve declared the int array allMessageTypeIDs, for simplicity. But remember that the values it holds may differ, so that’s why I’ve added the for loop. Maybe it’s “illegal” to use a Concat() in a loop that way, but I could not find any relevant information on this exception.

The class MyObject basically hold a int (NumberOfOccurences) and a ParsedMessage database object, nothing else.

Any suggestions on what could be wrong with my code that causes the exception?

Thanks
Francis

Answer 1

Never use the variable you’re looping with in your Linq queries. It just doesn’t work. You want to assign a new temporary variable to use instead.

IQueryable<MyObject> concatquery;
int[] allMessageTypeIDs = new int[] { 0, 1, 2 };
for (int mt = 0; mt < allMessageTypeIDs.Length; mt++)
{
    var myItem = allMessageTypeIDs[mt]; // <-- HERE!
    if (mt == 0)
    {
        concatquery = (from pm in db.ParsedMessages
                       join pml in db.ParsedMessageLinks on pm.ParsedMessageID equals pml.ParsedMessageID
                       where pml.AnotherID == 0 && pm.MessageTypeID == myItem
                       orderby pm.ParsedMessageID descending
                       select new MyObject
                       {
                           NumberOfOccurences = pml.Occurrences,
                           ParsedMessage = pm
                       }).Take(100);
    }
    else
    {
        concatquery = concatquery.Concat(from pm in db.ParsedMessages
                                        join pml in db.ParsedMessageLinks on pm.ParsedMessageID equals pml.ParsedMessageID
                                        where pml.AnotherID == 0 && pm.MessageTypeID == myItem
                                        orderby pm.ParsedMessageID descending
                                        select new MyObject
                                        {
                                            NumberOfOccurences = pml.Occurrences,
                                            ParsedMessage = pm
                                        }).Take(100);
    }
}
var results = concatquery.ToArray();