StreamInsight: Obscure LINQ error – The field ‘Key’ of type ‘IKey`1’ either contains a nested member, or is a type that is not supported as event type field


This is a pretty obscure one, and a nice short snippet of a blog post.  I ran into this error yesterday while building out some StreamInsight applications for real time web analytics, and was very briefly stumped.  Let’s have a quick peek at how I ran into it.

Starting with a working query, I needed a different pivot/aggregation on the same data stream.  My working query was:

Code Snippet
  1. // Create the "average" quality metrics for each video from raw video quality
  2. // metrics (i.e. not quantized)
  3. var videoQualityByVideo = from e in this.videoQualityStream
  4. group e by new { e.ApplicationId, e.VideoId } into videoGroups
  5. from win in videoGroups.TumblingWindow(defaultWindowSize,
  6.     HoppingWindowOutputPolicy.ClipToWindowEnd)
  7. select new
  8. {
  9.     ApplicationId = videoGroups.Key.ApplicationId,
  10.     VideoId = videoGroups.Key.VideoId,
  11.     AverageBitrate = win.Avg(e => e.BitRate)
  12. };

Pretty basic stuff – now, I wanted to group by another field (in this case a field called EdgeServerName).  So, I copy and paste, change the grouping field name and away we go:

Code Snippet
  1. // Create the "average" quality metrics for each video from raw video quality
  2. // metrics (i.e. not quantized)
  3. var videoQualityByEdgeServer = from e in this.videoQualityStream
  4. group e by new { e.EdgeServerName } into videoGroups
  5. from win in videoGroups.TumblingWindow(defaultWindowSize,
  6.     HoppingWindowOutputPolicy.ClipToWindowEnd)
  7. select new
  8. {
  9.     EdgeServerName = videoGroups.Key,
  10.     AverageBitrate = win.Avg(e => e.BitRate)
  11. };

Should be fine – right?  Those of you familiar with LINQ are probably already laughing at the minor syntax mistake I made, but for the rest of us here’s what happened when I tried to run the query:

Microsoft.ComplexEventProcessing.Linq.QueryGenerationException was unhandled
  Message=The field 'Key' of type 'IKey`1' either contains a nested member, or is a type that 
is not supported as event type field.  Source=Microsoft.ComplexEventProcessing.Diagnostics
  StackTrace:
Microsoft.ComplexEventProcessing.Diagnostics.Exceptions.Throw(Exception exception)       
at Microsoft.ComplexEventProcessing.Linq.GroupUnionSelectRewriter.VisitMemberAccess(MemberExpression memberExpr)
...

Ouch.  What did I do wrong?  A composite group key must have more than one field!

Right Wrong! Right Smile
Code Snippet
  1. group e by new { e.ApplicationId, e.VideoId } into videoGroups
Code Snippet
  1. group e by new { e.ApplicationId } into videoGroups
Code Snippet
  1. group e by e.ApplicationId into videoGroups

Removing the new {} syntax for a single grouping field worked perfectly.  Another bit of LINQ learned!

Code Snippet
  1. // Create the "average" quality metrics for each video from raw video quality
  2. // metrics (i.e. not quantized)
  3. var videoQualityByEdgeServer = from e in this.videoQualityStream
  4. group e by e.EdgeServerName into videoGroups
  5. from win in videoGroups.TumblingWindow(defaultWindowSize,
  6.     HoppingWindowOutputPolicy.ClipToWindowEnd)
  7. select new
  8. {
  9.     EdgeServerName = videoGroups.Key,
  10.     AverageBitrate = win.Avg(e => e.BitRate)
  11. };

But wait… there’s more!  Through the course of the same project, I also ran into another way to throw this error, in a slightly different fashion.

Code Snippet
  1. var videoQualityByEdgeServer = from e in this.videoQualityStream
  2.     group e by new { e.ApplicationId, e.EdgeServerName } into edgeGroups
  3.     from win in edgeGroups.TumblingWindow(videoQualityEventWindowSize, windowAlignment,
  4.         HoppingWindowOutputPolicy.ClipToWindowEnd)
  5.     select new
  6.     {
  7.         ApplicationId = edgeGroups.Key,
  8.         EdgeServerName = edgeGroups.Key.EdgeServerName,
  9.  
  10.         // Buffering Milliseconds -> Seconds
  11.         // Divide by sampling frequency seconds
  12.         // Multiply by 100 (1 / 1000 * 100 = 1 / 10)
  13.         AvgBitrate = win.Avg(e => e.BitRate)
  14.     };

If you have a composite key, and don’t use the individual properties of the grouping key, the error is thrown.  Easy in retrospect, right? Smile

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