将ActiveRecord查询转换为sql

Question

I need to write a migration to create some resources in a separate(new) table based on the existing records in another table

This is basically what I need to do in RoR:

  Note.all.each do |note|
    object_id = sprintf '%09d', note.id
    url_key = "notes/documents/#{object_id.scan(/.{3}/).join('/')}/original/#{note.document_file_name}"

    note.documents.create(s3_key: url_key)
  end

I need to loop through all the existing notes, and then create documents for them... ( as I'm migrating document from being a direct field on notes to a one_to_many association.)

How will this looks like in a sql query? (specifically a postgresql query)

Answer 1

What you're looking for is an INSERT statement that inserts values from a query, which is part of the standard SQL spec. The trickier part with your question is breaking up the ID field into subfolders, which I've used a CTE to do (but not particularly cleverly) in this solution:

WITH noteparts AS (
  SELECT 
    id,
    document_file_name,
    substr(to_char(id, 'FM000000000'), 1, 3) as part_one,
    substr(to_char(id, 'FM000000000'), 4, 3) as part_two,
    substr(to_char(id, 'FM000000000'), 7, 3) as part_three
  FROM notes
)
INSERT INTO documents (note_id, s3_key)
SELECT id, 'notes/documents/' || part_one || '/' || part_two || '/' || part_three || '/' || document_file_name
FROM noteparts;