如何使用 XUnit 在 Cosmos DB 中对 LINQ 查询进行单元测试

Question

    public async Task<List<Group>?> GetInfosforGroupIdAsync(string groupId)
    {
           Container? cosmosContainer =
           await database.CreateContainerIfNotExistsAsync("Group", "/GroupId");
           if (groupId == null)
              return null;
                
           var query = cosmosContainer.GetItemLinqQueryable<Group>()
                    .Where(d => d.GroupId == groupId);
           var queryResult = query.ToFeedIterator();
           if (queryResult.HasMoreResults)
           {
               FeedResponse<Group> currentResultSet = await queryResult.ReadNextAsync();
               List<DedupeGroup> result = currentResultSet.ToList<Group>();
               return result;
           }
           return null;
   }

How can I write units tests for the above method using xunit tests. Assume Group is like below

class Group{
  string GroupId;
  string GroupInfo;
}

Answer 1

That's generally not a good idea. If you really wanted to unit test the method as shown, you would need to create mockups of all the used Cosmos DB classes. This would be a lot of code for little value in return (or even negative net value since tests have to be maintained, may contain bugs etc.). But that's not the biggest problem.

You could factor out the thing that you actually want to unit test and only test that, namely the LINQ expression. You could test the IQueryable by mocking it with a normal in-memory IEnumerable . But that doesn't actually work in general because LINQ queries on an IEnumerable don't behave like LINQ queries against Cosmos DB. For example, the null-coalesce operator in C# ( ?? ) is translated into an undefined-coalesce operator in Cosmos DB ( ?? ), or you can access non-existing keys using an indexer in Cosmos DB without error ( c['foo'] ) but you get a KeyNotFoundException in-memory.

Unfortunately, there is no in-memory mock CosmosClient avaiable at the moment. So you would have to write your own LINQ provider that interprets LINQ queries exactly like Cosmos DB does, but in-memory, which would be a lot of work for very little value in return.