Convert an SQL query with row_number to AR code - postgresql

Question

I have somewhat complex SQL query that I need to convert to nice and clean AR code, and I'm having some troubles with it.
Here's the query:
SELECT a.*
FROM
fixed_assets a
INNER JOIN
(
SELECT e.id, e.fixed_asset_id
FROM
fixed_asset_book_entries e
WHERE e.book_id = %SOME_VALUE_1%
) e_mod
ON e_mod.fixed_asset_id = a.id
INNER JOIN
(
SELECT s.fixed_asset_book_entry_id,
s.status,
ROW_NUMBER() OVER (PARTITION BY s.fixed_asset_book_entry_id ORDER BY s.created_at DESC) AS rn
FROM
status_changes s
WHERE s.created_at < %SOME_VALUE_2%
) s_mod
ON s_mod.fixed_asset_book_entry_id = e_mod.id AND s_mod.rn = 1 AND s_mod.status <> 'inactive'
ORDER BY a.id;
So, the point of it all is to extract such fixed_assets rows, that have related fixed_asset_book_entries with certain book_id, and it's last status_change before certain date has any status except inactive.
What I want to end up with is a class-level (scope?) method FixedAsset.active_within_book_on_date(book_id, date), that will return FixedAsset objects, that comply with restrictions I've explained above. I'm familiar with joins method, but I'm not sure how to handle row_number function except passing raw SQL to joins call.

Answer 1

I think the best you can do is something like the following. In lib/sql_template.rb:
class SqlTemplate
attr_reader :sql
# Load the file and process the ERB
# Call it like this:
# sql = SqlTemplate.new(filename, binding)
def initialize(filename, the_binding)
raw_code = File.read(File.join(Rails.root, 'lib/sql', filename))
template = ERB.new(raw_code)
#sql = template.result(the_binding)
end
end
Then define your raw SQL in lib/sql/active_within_book_on_date.sql. Which would then allow you to do this:
class FixedAsset
def self.active_within_book_on_date(book_id, date)
template = SqlTemplate.new('active_within_book_on_date.sql', binding)
self.find_by_sql(template.sql)
end
end
Your SQL file would look like this:
SELECT a.*
FROM
fixed_assets a
INNER JOIN
(
SELECT e.id, e.fixed_asset_id
FROM
fixed_asset_book_entries e
WHERE e.book_id = <%=book_id%>
) e_mod
ON e_mod.fixed_asset_id = a.id
INNER JOIN
(
SELECT s.fixed_asset_book_entry_id,
s.status,
ROW_NUMBER() OVER (PARTITION BY s.fixed_asset_book_entry_id ORDER BY s.created_at DESC) AS rn
FROM
status_changes s
WHERE s.created_at < '<%=date%>'
) s_mod
ON s_mod.fixed_asset_book_entry_id = e_mod.id AND s_mod.rn = 1 AND s_mod.status <> 'inactive'
ORDER BY a.id;
That's probably as 'nice and clean' as you can get.