cosmos GetItemLinqQueryable with ItemResponse without partitionKey

I need to query container by Id parameter without knowing partition key also I need to retrieve ItemResponse of that in order to reach ETag property

Normally with partion key this is easy to do with partition key, like this:

var orderEntity = await container.ReadItemAsync<OrderEntity>(orderId, new PartitionKey(partitionKeyValue));

but how to do if you don't know value for partition key ?

I tried to do it this way

var query = container.GetItemLinqQueryable<ItemResponse<OrderEntity>>(requestOptions: new QueryRequestOptions()).AsQueryable();
query = query.Where(t => t.Resource.Id == orderId);

But this returns me null, if I execute query without where condition then I got 100 objects of <ItemResponse <OrderEntity> > but Resource property is empty for all of them

A ReadItem operation is reading an item by it's id property and Partition Key Value.

When querying without the Partition Key, you are doing a "cross partition query".

What you are looking for is something like:

var query = container.GetItemLinqQueryable<OrderEntity>(requestOptions: new QueryRequestOptions()).AsQueryable();
query = query.Where(t => t.Id == orderId);

Assuming that in OrderEntity, your Id property matches with an id serialization (you have for example [JsonProperty("id")]). If there is another property that matches with id, use that.

The main difference with your code in your question is that you don't need to use ItemResponse, that is just the type that ReadItem returns. In a query, you are querying over your items, so use the POCO/type that represents your items.

Remember that a cross partition query can take longer depending on the number of partitions in the container, and make sure you drain all pages, don't assume results will be always on the first page (if you are paging). See the examples on the API docs: https://learn.microsoft.com/dotnet/api/microsoft.azure.cosmos.container.getitemlinqqueryable?view=azure-dotnet