It seems like you'd have to do it in Ruby if you want consistent results across database types, as the database itself interprets whether or not the NULLS go at the front or end of the list.

Photo.all.sort {|a, b| a.collection_id.to_i <=> b.collection_id.to_i}

But that is not very efficient.

Adding arrays together will preserve order:

@nonull = Photo.where("collection_id is not null").order("collection_id desc")
@yesnull = Photo.where("collection_id is null")
@wanted = @nonull+@yesnull

http://www.ruby-doc.org/core/classes/Array.html#M000271

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 last
Photo.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 Last
Photo.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')

The easiest way is to use:

.order('name nulls first')

Put minus sign in front of column_name and reverse the order direction. It works on mysql. More details

Product.order('something_date ASC') # NULLS came first
Product.order('-something_date DESC') # NULLS came last

Bit late to the show but there is a generic SQL way to do it. As usual, CASE to the rescue.

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

For posterity's sake, I wanted to highlight an ActiveRecord error relating to NULLS FIRST.

If you try to call:

Model.scope_with_nulls_first.last

Rails will attempt to call reverse_order.first, and reverse_order is not compatible with NULLS LAST, as it tries to generate the invalid SQL:

PG::SyntaxError: ERROR:  syntax error at or near "DESC"
LINE 1: ...dents"  ORDER BY table_column DESC NULLS LAST DESC LIMIT...

This was referenced a few years ago in some still-open Rails issues (one, two, three). I was able to work around it by doing the following:

  scope :nulls_first, -> { order("table_column IS NOT NULL") }
scope :meaningfully_ordered, -> { nulls_first.order("table_column ASC") }

It appears that by chaining the two orders together, valid SQL gets generated:

Model Load (12.0ms)  SELECT  "models".* FROM "models"  ORDER BY table_column IS NULL DESC, table_column ASC LIMIT 1

The only downside is that this chaining has to be done for each scope.

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.

In my case I needed sort lines by start and end date by ASC, but in few cases end_date was null and that lines should be in above, I used

@invoice.invoice_lines.order('start_date ASC, end_date ASC NULLS FIRST')

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.

Here are some Rails 6 solutions.

The answer by @Adam Sibik is a great summary about the difference between various database systems.

Unfortunately, though, some of the presented solutions, including "Universal solution" and "PostgreSQL and MySQL specific", would not work any more with Rails 6 (ActiveRecord 6) as a result of its changed specification of order() not accepting some raw SQLs (I confirm the "PostgreSQL specific" solution still works as of Rails 6.1.4). For the background of this change, see, for example,
"Updates for SQL Injection in Rails 6.1" by Justin.

To circumvent the problem, you can wrap around the SQL statements with Arel.sql as follows, where NULLs come last, providing you are 100% sure the SQL statements you give are safe.

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

Just for reference, if you want to sort by a Boolean column (is_ok, as an example) in the order of [TRUE, FALSE, NULL] regardless of the database systems, either of these should work:

Photo.order(Arel.sql('CASE WHEN is_ok IS NULL THEN 1 ELSE 0 END, is_ok DESC'))
Photo.order(Arel.sql('CASE WHEN is_ok IS NULL THEN 1 WHEN is_ok IS TRUE THEN -1 ELSE 0 END'))

(n.b., SQLite does not have the Boolean type and so the former may be safer arguably, though it should not matter because Rails should guarantee the value is either 0 or 1 (or NULL).)

Rails 6.1 adds nulls_first and nulls_last methods to Arel for PostgreSQL.

Example:

User.order(User.arel_table[:login_count].desc.nulls_last)

Source: https://www.bigbinary.com/blog/rails-6-1-adds-nulls-first-and-nulls-last-to-arel