Abstract Azure IMobileServiceTable<T> behind repository - azure

Question

I want my repsository to be independent of the data access technology. Currently I am working on a Xamrin.Forms App that uses Azure Mobile App Services for data access. For performance and flexibility reasons I want my repository to look simmilar like the following:
Task<IEnumerable<IDomainObject>> GetDomainObjectAsync(Func<IQueryable<IDomainObject>, IQueryable<IDomainObject>> query)
Suppose my IDomainObject looks like the following:
public interface IDomainObject
{
string Name { get; }
}
and my DataAccess Object:
internal class AzureDomainObject : IDomainObject
{
public string Name { get; set; }
public string Id { get; set; }
}
As far as I found out and tested I can do the following to query the database within my repository implementation:
public async Task<IEnumerable<IDomainObject>> GetDomainObjectAsync(Func<IQueryable<IDomainObject>, IQueryable<IDomainObject>> query)
{
// _table of type IMobileServiceTable<AzureDomainObject> gotten by MobileServiceClient
var tableQuery = _table.GetQuery();
tableQuery.Query = tableQuery.Query.Take(4); // 1) this was for testing and it works (ordering etc also works)
// tableQuery.Query = query(tableQuery.Query); // 2) this was my initial idea how to use the input param
await _table.ReadAsync(tableQuery);
}
My poblem now is how to use the input param query to replace 1) with 2).
tableQuery.Query expects an IQueryable<AzureDomainObject> but query is of type IQueryable<IDomainObject>.
Neither .Cast<AzureDomainObject>() nor .OfType<AzureDomainObject>() work to convert. Nor does (IQueryable<IAzureDomainObject>)query; work.
Cast and OfType throw NotSupportedException and the hard cast throws an InvalidCastException.
I also tried to extract the Expression from the query input param and assign it to the tableQuery.Query. But then a runtime exception occurs that they are not compatible.
Another idea I had was to use the ReadAsync(string) overload and pass the string representation of the passed query param. But this way I don't know how to generate the string.
So the final question is: Does anyone knows how to hide both AzureDomainObject and IMobileServiceTable from the domain model but keep the flexibility and performance benefits of IQueryable in the repository interface?

Answer 1

According to your description, I checked this issue and here is my implementation for this scenario, you could refer to them.
Model:
public class TodoItem : IDomainObject
{
public string Id { get; set; }
[JsonProperty(PropertyName = "text")]
public string Text { get; set; }
[JsonProperty(PropertyName = "complete")]
public bool Complete { get; set; }
}
public interface IDomainObject
{
string Id { get; set; }
}
Repository:
public interface IAzureCloudTableRepository<T> where T : IDomainObject
{
Task<IEnumerable<T>> GetDomainObjectAsync(Func<IQueryable<T>, IQueryable<T>> query);
}
public class AzureCloudTableRepository<T> : IAzureCloudTableRepository<T> where T : IDomainObject
{
IMobileServiceTable<T> table;
public AzureCloudTableRepository(MobileServiceClient client)
{
this.table = client.GetTable<T>();
}
public async Task<T> CreateItemAsync(T item)
{
await table.InsertAsync(item);
return item;
}
public async Task<IEnumerable<T>> GetDomainObjectAsync(Func<IQueryable<T>, IQueryable<T>> query)
{
var tableQuery = this.table.CreateQuery();
tableQuery.Query = tableQuery.Query.Take(4); //the internal fixed query
tableQuery.Query = query(tableQuery.Query); //the external query
return await tableQuery.ToEnumerableAsync();
}
}
TEST:
var mobileService = new MobileServiceClient("https://{your-app-name}.azurewebsites.net");
var todoitem = new AzureCloudTableRepository<TodoItem>(mobileService);
var items = await todoitem.GetDomainObjectAsync((query) =>
{
return query.Where(q => q.Text!=null);
});