I'm no expert at SQL, but why not just sort by if something is null first then sort by how you wanted to sort it.

Photo.order('collection_id IS NULL, collection_id DESC')  # Null's lastPhoto.order('collection_id IS NOT NULL, collection_id DESC') # Null's first

If you are only using PostgreSQL, you can also do this

Photo.order('collection_id DESC NULLS LAST')  #Null's LastPhoto.order('collection_id DESC NULLS FIRST') #Null's First

If you want something universal (like you're using the same query across several databases, you can use (courtesy of @philT)

Photo.order('CASE WHEN collection_id IS NULL THEN 1 ELSE 0 END, collection_id')

Even though it's 2017 now, there is still yet to be a consensus on whether NULLs should take precedence. Without you being explicit about it, your results are going to vary depending on the DBMS.

The standard doesn't specify how NULLs should be ordered in comparison with non-NULL values, except that any two NULLs are to be considered equally ordered, and that NULLs should sort either above or below all non-NULL values.

source, comparison of most DBMSs

To illustrate the problem, I compiled a list of a few most popular cases when it comes to Rails development:

PostgreSQL

NULLs have the highest value.

By default, null values sort as if larger than any non-null value.

source: PostgreSQL documentation

MySQL

NULLs have the lowest value.

When doing an ORDER BY, NULL values are presented first if you do ORDER BY ... ASC and last if you do ORDER BY ... DESC.

source: MySQL documentation

SQLite

NULLs have the lowest value.

A row with a NULL value is higher than rows with regular values in ascending order, and it is reversed for descending order.

source

Solution

Unfortunately, Rails itself doesn't provide a solution for it yet.

PostgreSQL specific

For PostgreSQL you could quite intuitively use:

Photo.order('collection_id DESC NULLS LAST') # NULLs come last

MySQL specific

For MySQL, you could put the minus sign upfront, yet this feature seems to be undocumented. Appears to work not only with numerical values, but with dates as well.

Photo.order('-collection_id DESC') # NULLs come last

PostgreSQL and MySQL specific

To cover both of them, this appears to work:

Photo.order('collection_id IS NULL, collection_id DESC') # NULLs come last

Still, this one does not work in SQLite.

Universal solution

To provide cross-support for all DBMSs you'd have to write a query using CASE, already suggested by @PhilIT:

Photo.order('CASE WHEN collection_id IS NULL THEN 1 ELSE 0 END, collection_id')

which translates to first sorting each of the records first by CASE results (by default ascending order, which means NULL values will be the last ones), second by calculation_id.

Photo.order('collection_id DESC NULLS LAST')

I know this is an old one but I just found this snippet and it works for me.