How can I use LINQ in CosmosDB SDK v3.0 async query?

Question

I've been following the official documentation here: https://learn.microsoft.com/en-us/azure/cosmos-db/sql-api-get-started#Query

But I can't figure out how to correctly use a LINQ expression instead on an SQL string. I experimented with GetItemLinqQueryable, but I don't know if is the right way to use it. Also is not async.

var db = Client.GetDatabase(databaseId);
var container = db.GetContainer(containerId);

var q = container.GetItemLinqQueryable<Person>();
var result = q.Where(p => p.Name == "Name").ToList();

Is this the right way to use LINQ with Cosmos v3, and how to make it async?

score 36 · Accepted Answer · answered Jul 23 '19 at 20:24

36

You would use ToFeedIterator() and FeedIterator<T>.ReadNextAsync().

var db = Client.GetDatabase(databaseId);
var container = db.GetContainer(containerId);

var q = container.GetItemLinqQueryable<Person>();
var iterator = q.Where(p => p.Name == "Name").ToFeedIterator();
var results = await iterator.ReadNextAsync();

answered Jul 23 '19 at 20:24

Chris Anderson

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9

BTW, ToFeedIterator() can be found in Microsoft.Azure.Cosmos.Linq – Louis S. Berman Nov 01 '19 at 21:04
5

you can find an example here https://learn.microsoft.com/en-us/dotnet/api/microsoft.azure.cosmos.container.getitemlinqqueryable?view=azure-dotnet – R.Titov Dec 18 '19 at 12:36
16

Wish they would just support ToListAsync (or ToArrayAsync) and IAsyncEnumerable – James Hancock Feb 19 '20 at 15:45
I wrote a small extension to get an IAsyncEnumerable that you can call ToListAsync on here https://stackoverflow.com/questions/67317049/how-to-convert-cosmosdb-feediterator-result-to-iasyncenumerable-or-ienumerable/67317050#67317050 – Jonas Stensved Apr 29 '21 at 12:08

score 12 · Answer 2 · answered Oct 28 '20 at 11:44

If your application follows a layered architecture and you'd like to give your domain layer full control over the query then it's possible to wrap cosmos IQueryable<Person> with a custom IQueryProvider that implements IAsyncEnumerable e.g.

By doing that you can hide the implementation details of asynchronously iterating over the result from your domain layer.

Persistence layer

public class PersonRepository
{
    public IQueryable<Person> Persons => _cosmosContainer.GetItemLinqQueryable<Person>().ToCosmosAsyncQueryable();
}

Domain layer

var persons = await _personRepository.Persons
    .Where(p => p.Name == "Name")
    .AsAsyncQueryable()
    .ToListAsync(cancellationToken);

ToListAsync is available from System.Linq.Async that can be referenced from your domain layer

Domain layer extensions

public static IAsyncEnumerable<T> AsAsyncQueryable<T>(this IQueryable<T> queryable)
{
    return (IAsyncEnumerable<T>)queryable;
}

Persistence layer extensions

internal static class CosmosAsyncQueryableExtensions
{
    internal static IQueryable<T> ToCosmosAsyncQueryable<T>(this IOrderedQueryable<T> source)
    {
        return new CosmosAsyncQueryable<T>(source);
    }
}

internal class CosmosAsyncQueryable<TResult> : IEnumerable<TResult>, IQueryable<TResult>, IAsyncEnumerable<TResult>
{
    private readonly IQueryable<TResult> _queryable;

    public CosmosAsyncQueryable(IQueryable<TResult> queryable)
    {
        _queryable = queryable;
        Provider = new CosmosAsyncQueryableProvider(queryable.Provider);
    }

    public Type ElementType => typeof(TResult);

    public Expression Expression => _queryable.Expression;

    public IQueryProvider Provider { get; }

    public IEnumerator<TResult> GetEnumerator() => _queryable.GetEnumerator();

    IEnumerator IEnumerable.GetEnumerator() => _queryable.GetEnumerator();

    public async IAsyncEnumerator<TResult> GetAsyncEnumerator(CancellationToken cancellationToken = default)
    {
        var iterator = _queryable.ToFeedIterator();

        while (iterator.HasMoreResults)
        {
            foreach (var item in await iterator.ReadNextAsync(cancellationToken))
            {
                yield return item;
            }
        }
    }
}

internal class CosmosAsyncQueryableProvider : IQueryProvider
{
    private readonly IQueryProvider _provider;

    public CosmosAsyncQueryableProvider(IQueryProvider provider) => _provider = provider;

    public IQueryable<TElement> CreateQuery<TElement>(Expression expression) =>
        new CosmosAsyncQueryable<TElement>(_provider.CreateQuery<TElement>(expression));

    public IQueryable CreateQuery(Expression expression) => CreateQuery<object>(expression);

    public object Execute(Expression expression) => _provider.Execute(expression);

    public TResult Execute<TResult>(Expression expression) => _provider.Execute<TResult>(expression);
}

Hi according to https://stackoverflow.com/questions/38402824/multiple-queries-in-async-await-error-ienumerable-does-not-contain-tolistasync once you convert to IEnumerable you loose ability to use ToList? — Kentonbmax, Aug 16 '22 at 15:17

How can I use LINQ in CosmosDB SDK v3.0 async query?

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