SelectMany is a simple operator, but can be difficult to grasp for beginners. For more information on how SelectMany works in LINQ, see the official Microsoft documentation.

One important difference in the implementation of SelectMany in LINQ to Signum compared to LINQ to SQL is that LINQ to SQL tries to reduce CROSS APPLY operations into CROSS JOIN opportunistically. Signum LINQ does not perform this optimization. Instead, it always uses CROSS APPLY whenever a SelectMany appears.

For example, consider the following query:

IQueryable<CommentEntity> result = Database.Query<BugEntity>().SelectMany(b => b.Comments);

// Or the equivalent using query expressions
IQueryable<CommentEntity> result = from b in Database.Query<BugEntity>()
                                   from c in b.Comments    
                                   select c;

This will always be translated to a CROSS APPLY in SQL:

SELECT s1.Text, s1.Date, cdn.Id, cdn.Ticks, cdn.Name, s1.WriterId_Customer, ddn.Id AS Id1, ddn.Ticks AS Ticks1, ddn.Name AS Name1, s1.WriterId_Developer, s1.HasValue
FROM BugEntity AS bdn
CROSS APPLY (
  SELECT bdnc.Text, bdnc.Date, bdnc.WriterId_Customer, bdnc.WriterId_Developer, bdnc.HasValue
  FROM BugDNComments AS bdnc
  WHERE bdn.Id = bdnc.ParentId
) AS s1
LEFT OUTER JOIN CustomerEntity AS cdn
  ON s1.WriterId_Customer = cdn.Id
LEFT OUTER JOIN DeveloperEntity AS ddn
  ON s1.WriterId_Developer = ddn.Id

If you want LINQ to Signum to work on other DBMSs, such as PostgreSQL, MySQL, or Oracle, this behavior may present challenges, as not all databases support CROSS APPLY.

The second difference is not just an implementation detail:

Signum LINQ supports OUTER APPLY by adding DefaultIfEmpty at the end of the selector lambda.

For example:

IQueryable<CommentEntity> result = from b in Database.Query<BugEntity>()
                                   from c in b.Comments.DefaultIfEmpty()    
                                   select c;

This will return one null element for each BugEntity that has no Comments, instead of skipping it. This is achieved by using OUTER APPLY instead of CROSS APPLY:

SELECT s1.Text, s1.Date, cdn.Id, cdn.Ticks, cdn.Name, s1.WriterId_Customer, ddn.Id AS Id1, ddn.Ticks AS Ticks1, ddn.Name AS Name1, s1.WriterId_Developer, s1.HasValue
FROM BugEntity AS bdn
OUTER APPLY (
  SELECT bdnc.Text, bdnc.Date, bdnc.WriterId_Customer, bdnc.WriterId_Developer, bdnc.HasValue
  FROM BugDNComments AS bdnc
  WHERE bdn.Id = bdnc.ParentId
) AS s1
LEFT OUTER JOIN CustomerEntity AS cdn
  ON s1.WriterId_Customer = cdn.Id
LEFT OUTER JOIN DeveloperEntity AS ddn
  ON s1.WriterId_Developer = ddn.Id