ASP.NET Core & EntityFramework Core: Left (Outer) Join in Linq

Question

I am trying to get a left join working in Linq using ASP.NET Core and EntityFramework Core.

Simple situation with two tables:

Person (id, firstname, lastnam

Answer 1

If you need to do the Left joins then you have to use into and DefaultIfEmpty() as shown below.

var result = from person in _dbContext.Person
             join detail in _dbContext.PersonDetails on person.Id equals detail.PersonId into Details
             from m in Details.DefaultIfEmpty()
               select new
                {
                    id = person.Id,
                    firstname = person.Firstname,
                    lastname = person.Lastname,
                    detailText = m.DetailText
                };

You can learn more about it : Left Outer Join in LINQ to Entities

Answer 2

You not doing left join, the linq which you are using is basically creating a inner join. For left join in linq use into keyword

[HttpGet]
public IActionResult Get()
{
    var result = from person in _dbContext.Person
                    join detail in _dbContext.PersonDetails on person.Id equals detail.PersonId  
                    into Details
                    from defaultVal in Details.DefaultIfEmpty()
                    select new
                    {
                        id = person.Id,
                        firstname = person.Firstname,
                        lastname = person.Lastname,
                        detailText = defaultVal.DetailText
                    };
   return Ok(result);
}

Answer 3

I agree to author of this post - it is still look like a bug! If you have empty joined tables you always receive "Object reference not set to an instance of an object.". The only way is to check joined tables to null:

    IEnumerable<Models.Service> clubServices =
        from s in services
        from c in clubs.Where(club => club.ClubId == s.ClubId).DefaultIfEmpty()
        from t in clubs.Where(tenant => tenant.TenantId == c.TenantId).DefaultIfEmpty()
        select new Models.Service
        {
            ServiceId = s.ServiceId.ToString(),
            ClubId = c == null ? (int?)null : c.ClubId,
            ClubName = c == null ? null : c.Name,
            HasTimeTable = s.HasTimeTable,
            MultipleCount = s.MultipleCount,
            Name = s.Name,
            Tags = s.Tags.Split(';', StringSplitOptions.RemoveEmptyEntries),
            TenantId = t == null ? (int?)null : t.TenantId,
            TenantName = t == null ? null : t.Name
        };

I unable to check "detailText = person.PersonDetails.Select(d => d.DetailText).SingleOrDefault()" because my joined tables are in different DB.