Many to many with dataLoader in graphql-dotnet

Question

How can we achieve many to many with dataLoader?

Lets say i want to fetch the companies with there contacts (and a contact can have multiple companies). I have a companyLinks as midde table (with companyId/contactId).

I have this which works when there is only 1 contact for a company:

public async Task<ILookup<Guid, Contact>> GetContactsFromCompanyBatchAsync(IEnumerable<Guid> companyIds)    
{
  var contacts = _entities.Include(c => c.CompanyLinks).Where(contact => contact.CompanyLinks.Any(cl => companyIds.Contains(cl.CompanyId)));
  return contacts.ToLookup(cont => {
       var res = cont.CompanyLinks.Any() ? cont.CompanyLinks.ElementAt(0).CompanyId : Guid.Empty;
       return res;
  });
}

But what if a contact has more then 1 company? How can i know the correct companyId ?

Answer 1

Thanks to sungram3r for the hint. Solution is to start from the joining-table.

public async Task<ILookup<Guid, Contact>> GetContactsFromCompanyBatchCollectionAsync(IEnumerable<Guid> companyIds)
    {
        var companyContacts = _salesNoteContext.CompanyContact
           .Where(contactCompany => companyIds.Contains(contactCompany.CompanyId))
           .Include(contactCompany => contactCompany.Company);
        return companyContacts.ToLookup(contactCompany => contactCompany.CompanyId, contactCompany => contactCompany.Contact);
    }