Cosmos DB Client V3

 The latest Cosmos DB V3 client library makes coding easier for .NET developers. The class model is simpler and more sensible, and the LINQ extensions let you write concise and type-safe queries.

In the old library, to get the Id and Name of every comedy CD in my library I would need to construct a query string like the following and pass it to a long complicated database query call:

SELECT c.Id, c.Name FROM c
  WHERE c.Type=2 AND
      c.Media="CD" AND
      ARRAY_CONTAINS(c.Genres, {Name:"Comedy"}, true)

I was using nameof() to help strongly-type the resulting string (not shown), but it was rather clumsy and verbose. Now I can code a strongly-typed LINQ statement with intellisense:

var query = container.GetItemLinqQueryable<Title>()
    .Where(t => t.Type == DocumentType.Title &&
        t.Media == "CD" &&
        t.Genres.Any(g => g.Name == "Comedy"))
    .Select(t => new { t.Id, t.Name });

The LINQ query expression tree is converted into an actual database query and the results by:

var feed = query.ToFeedIterator();
return await IterateFeedResults(feed);

The last method is a helper I made up, and it's a great practical example of how you can turn a loop of async calls into a return value which is a convenient async enumerable sequence:

async IAsyncEnumerable<T> IterateFeedResults<T>(FeedIterator<T>> feed)
{
  while (feed.HasMoreResults)
  {
    foreach (var item in await feed.ReadNextAsync())
    {
      yield return item;
    }
  }
}

The logical structure of a Cosmos database is now more closely represented by the classes. The preamble to running queries is simply this:

var client = new CosmosClient(uri, dbkey);
var database = client.GetDatabase("MyDatabase");
var container = database.GetContainer("MyContainer");
var query = container.GetItemLinqQueryable(...);

For more information about how I enjoy using document databases like Cosmos DB, see my old blog post titled Collections Database History.

Orthogonal Programming