How can we improve Azure Cosmos DB?

Add support for single entity retrieval instead of IEnumarable

Single \ First and so are currently not supported in Document DB

For example, the following code:

return _client.CreateDocumentQuery<TEntity>(_collection.SelfLink).SingleOrDefault(x => x.Id == id);

causes the error:

An exception of type 'Microsoft.Azure.Documents.Linq.DocumentQueryException' occurred in Microsoft.Azure.Documents.Client.dll but was not handled in user code

Additional information: Query expression is invalid, expression return type Entities.Models.Hierarchy is unsupported. Query must evaluate to IEnumerable.

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Doron Grinzaig shared this idea  ·   ·  Flag idea as inappropriate…  ·  Admin →

Thank you for your feedback. I have discussed this in more detail with our team.

You are correct the workaround suggested to use .AsEnumerable().FirstOrDefault(). should not be a recommended work around. This way results in materializing all documents on the client first before getting the 1st document, which is not very efficient and the exact opposite of what you are trying to achieve.

Instead we recommend you use Take(1).AsEnumerable() and then .First() or .Single() or .FirstOrDefault() for Single() and First(). Take(1) is translated to SELECT TOP 1 and is processed server-side so more efficient than the previous suggestion and is what you are trying to achieve.

As to the original ask. The support for these operations can be done, but this work is not prioritized against the other work we want to deliver. I will move this back as unplanned but please note this is on our roadmap.

Thanks again for your suggestion and everyone’s feedback.


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  • Zach commented  ·   ·  Flag as inappropriate

    This is a simple request yet it's not being prioritized. Why can't we expect a modern DB to handle such requests? Please work on this.

  • Matthew Paul commented  ·   ·  Flag as inappropriate

    Closing this off by suggesting '.AsEnumerable().FirstOrDefault()' is a precisely the hack we're wanting to avoid. Can you reopen?

  • Brandon commented  ·   ·  Flag as inappropriate

    I ran into this too. A reasonable (though annoying) workaround is

    .Where(x => x.Id == id).AsEnumerable().SingleOrDefault()

    and this can be rewrapped as an extension method

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