C# DbDataReader double loop

Question

I am trying to create XML with the following structure:

<ResponseSubschemas>
    <organism>LMO</organism>
    <Subschema>
        <id>MLST</id>
        <name>MLST</name>
        <loci>
            <locus>LMO0558</locus>
            <locus>LMO0563</locus>
        </loci>
    </Subschema>
    <Subschema>
        <id>MLVST</id>
        <name>MLVST</name>
        <loci>
            <locus>LMO1305</locus>
            <locus>LMO1089</locus>
        </loci>
    </Subschema>
</ResponseSubschemas>

I have a.o. two tables in my database:

• subschema with as columns ID and Name
• subschemamembers with as columns subschemaID and locusID.

I have the following C# code:

        XElement rootNode = new XElement("ResponseSubschemas", new XElement("organism", organismID));

        DbDataReader subschemaReader = conn.Query("SELECT ID, Name FROM subschema WHERE OrganismID = ?", organismDbID);
        while (subschemaReader.Read())
        {
            string subschemaDbID = (string)subschemaReader["ID"];
            XElement subschemaNode = new XElement("Subschema",
                new XElement("id", subschemaDbID),
                new XElement("name", subschemaReader["Name"])
            );
            rootNode.Add(subschemaNode);


        }

        responseXml = rootNode.ToString();

Which creates the following partial XML:

<ResponseSubschemas>
  <organism>LMO</organism>
  <Subschema>
    <id>MLST</id>
    <name>MLST</name>
  </Subschema>
  <Subschema>
    <id>MLVST</id>
    <name>MLVST</name>
  </Subschema>
</ResponseSubschemas>

Now I need to add for each Subschema node a loci node, loop over the loci and add them. I tried creating a new DbDataReader inside the while loop, but then i got errors like: There is already an open DataReader associated with this Connection which must be closed first. So, how do I do this?

Update:

Trying to go with the JOIN approach, I have crafted the following SQL:

SELECT subschema.Id, subschema.name as subschemaName, locus.Name as locusName
FROM subschema 
LEFT JOIN subschemamembers ON subschemamembers.SubSchemaID = subschema.PrimKey
LEFT JOIN locus ON subschemamembers.LocusID = locus.ID
WHERE  subschema.OrganismID = 6
ORDER BY subschema.Id;

Which gives the following output:

Id      subschemaName  locusName
MLST    MLST           LMO0558
MLST    MLST           LMO0563
MLVST   MLVST          LMO1305
MLVST   MLVST          LMO1089

but I still can't figure out how I would keep track of "whether the current db row points to a different subschemaDbID".

Answer 1

The easy way out that will make your solution work is to enable MARS (Multiple Active Result Sets): http://technet.microsoft.com/en-us/library/ms131686.aspx

To do it without enabling MARS, you would have to make a JOIN query fetching both subschema and subschemamembers and inside your XML creation code you need to keep track of whether the current db row points to a different subschemaDbID.