Filter on [email protected] about frost-server HOT 2 OPEN

Unfortunately this is (currently) not possible. Those counts are not actually properties of the Datastreams, they are calculated items of the Expand. This is also shown by the fact that the count depends on the filter that is used in the expand.
These two queries would give different results:

/v1.0/Datastreams?$expand=Observations($filter=result gt 5)&[email protected] gt 5
/v1.0/Datastreams?$expand=Observations&[email protected] gt 5

I think the best way to get the result you are looking for is to use a stored procedure. The branch moreProperties adds a properties field to each entity. Using that branch, you could use a stored procedure to store the observation count of a Datastream in this properties field of that Datastream. Then you can use that in your query.
This also has the advantage of being much faster, since the count operation can be very slow with large amounts of Observations.

from frost-server.

OData does specify this so it may be an interesting future extension.

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