Question

I’m trying to use MongoDB to cache some data for me, but I can’t seem to get the code to return a specific collection. I can get other collections without an issue, but for this one cannot get it to work and I have no idea why. The only error that I get is:

Ambiguous discriminator 'revamp@904'

I have searched long and hard for any indication as to what this means. Here’s the code I’m using:

let GetCacheDataObject ctxName cacheName collection = 
    let ctx = new DataContext(ctxName)
    let q = Query.EQ("CacheName", BsonValue.Create(cacheName.ToString()))
    let entity =
        match ctx.Db.CollectionExists(collection) with
        | false -> null
        | _ -> ctx.Db.GetCollection(typeof<DataObject>, collection).FindOneAs<DataObject>(q)
    entity

Once it his the ‘FindAsOne’, this error gets thrown.

The DataObject is a very basic custom object to hold data. Here’s the definition:

public class DataObject:IHaveIdentifier
{
    public BsonObjectId _id { get; set; }
    //public long Id { get; set; }
    public string[] Columns { get; set; }
    public IEnumerable<object[]> Rows { get; set; }
    public string CacheName { get; set; }
    public int GetColumnIndex(string column)
    {
        for (int i = 0; i < this.Columns.Length; i++)
            if (this.Columns[i] == column)
                return i;

        return -1;
    }
}

And the IHaveIdentifier interface is pretty basic:

public interface IHaveIdentifier
{
    BsonObjectId _id { get; set; }
}

Here’s the code that was used to save the data in the first place:

member x.Save<'T when 'T :> IHaveIdentifier>(entity:'T, collection:string) =
        if (entity._id = null ) then x.Insert<'T>(entity, collection)
        else
            x.Delete(entity, collection)
            x.Insert<'T>(entity, collection)
        x.VerifyNoErrors()

let CacheDataObject<'T when 'T :> IHaveIdentifier>(entity:'T, ctxName, collection) =
    let ctx = new DataContext(ctxName)
    ctx.Save(entity, collection)

This code was working the other day then something changed and I cannot seem to figure out what is going on.

UPDATE: Added initial saving code above

Answer 1

Brian and Robert,
Your input helped me figure out exactly what was going on. I was able to determine that the IEnumberable Rows property was exactly the issue. I was setting the Rows to a seq. In F# Seq are lazily evaluated, which in this case meant that Rows was being set to the output value (kind of like a ref) of the function, not the actual result. Once I converted the Rows using Seq.ToArray, then everything worked as expected.

Thanks you both so much for pointing me down the right path.