Question

I have this code, but i cant make it work:

images = Image.find_by_sql('PREPARE stmt FROM \' SELECT * FROM images AS i WHERE i.on_id = 1 AND i.on_type = "profile" ORDER BY i.updated_at LIMIT ?, 6\'; SET @lower_limit := ((5 DIV 6) * 6);  EXECUTE stmt USING @lower_limit;')

I got this error:

Mysql::Error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ‘SET @lower_limit := ((5 DIV 6) * 6); EXECUTE stmt USING @lower_limit’ at line 1: PREPARE stmt FROM ‘ SELECT * FROM images AS i WHERE i.on_id = 1 AND i.on_type = “profile” ORDER BY i.updated_at LIMIT ?, 6’; SET @lower_limit := ((5 DIV 6) * 6); EXECUTE stmt USING @lower_limit;

but if i use a sql app, like this, it works:

PREPARE stmt FROM ' SELECT * FROM images AS i WHERE i.on_id = 1 AND i.on_type = "profile" ORDER BY i.updated_at LIMIT ?, 6'; SET @lower_limit := ((5 DIV 6) * 6);  EXECUTE stmt USING @lower_limit;

SOLVED:
This generates 2 queries and is worse, but now i can get the offset of the image.
The other way around was with just one query, but i would not got any offset of the image, and anyway I couldn’t make it work.

def self.get_image_offset(id)
    image_offset = Image.find_by_sql("SELECT COUNT(id) as pos FROM images WHERE updated_at <= (SELECT updated_at FROM images WHERE id = #{id})")[0].pos.to_i
  end

  def self.get_group_offset(id, per_block, image_offset = nil)
    image_offset ||= Image.get_image_offset(id)
    group_offset = (image_offset / per_block).floor * per_block
    {:image_offset => image_offset, :group_offset => group_offset, :group_number => ( group_offset + per_block ) / per_block}
  end

Answer 1

You will be better off using something like execute [1] if you are writing the whole sql yourself (which is not really the ‘Rails way’, but that’s a whole other story all together)

[1] http://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/DatabaseStatements.html#M001934