Update values in each row based on foreign_key value - sql

Question

Downloads table:
id (primary key)
user_id
item_id
created_at
updated_at
The user_id and item_id in this case are both incorrect, however, they're properly stored in the users and items table, respectively (import_id for in each table). Here's what I'm trying to script:
downloads.each do |download|
user = User.find_by_import_id(download.user_id)
item = item.find_by_import_id(download.item_id)
if user && item
download.update_attributes(:user_id => user.id, :item.id => item.id)
end
end
So,
look up the user and item based on
their respective "import_id"'s. Then
update those values in the download record
This takes forever with a ton of rows. Anyway to do this in SQL?

Answer 1

If I understand you correctly, you simply need to add two sub-querys in your SELECT statement to lookup the correct IDs. For example:
SELECT id,
(SELECT correct_id FROM User WHERE import_id=user_id) AS UserID,
(SELECT correct_id FROM Item WHERE import_id=item_id) AS ItemID,
created_at,
updated_at
FROM Downloads
This will translate your incorrect user_ids to whatever ID you want to come from the User table and it will do the same for your item_ids. The information coming from SQL will now be correct.
If, however, you want to update the tables with the correct information, you could write this like so:
UPDATE Downloads
SET user_id = User.user_id,
item_id = Item.item_id
FROM Downloads
INNER JOIN User ON Downloads.user_id = User.import_id
INNER JOIN Item ON Downloads.item_id = Item.import_id
WHERE ...
Make sure to put something in the WHERE clause so you don't update every record in the Downloads table (unless that is the plan). I rewrote the above statement to be a bit more optimized since the original version had two SELECT statements per row, which is a bit intense.
Edit:
Since this is PostgreSQL, you can't have the table name in both the UPDATE and the FROM section. Instead, the tables in the FROM section are joined to the table being updated. Here is a quote about this from the PostgreSQL website:
When a FROM clause is present, what essentially happens is that the target table is joined to the tables mentioned in the fromlist, and each output row of the join represents an update operation for the target table. When using FROM you should ensure that the join produces at most one output row for each row to be modified. In other words, a target row shouldn't join to more than one row from the other table(s). If it does, then only one of the join rows will be used to update the target row, but which one will be used is not readily predictable.
http://www.postgresql.org/docs/8.1/static/sql-update.html
With this in mind, here is an example that I think should work (can't test it, sorry):
UPDATE Downloads
SET user_id = User.user_id,
item_id = Item.item_id
FROM User, Item
WHERE Downloads.user_id = User.import_id AND
Downloads.item_id = Item.import_id
That is the basic idea. Don't forget you will still need to add extra criteria to the WHERE section to limit the rows that are updated.

Answer 2

i'm totally guessing from your question, but you have some kind of lookup table that will match an import user_id with the real user_id, and similarly from items. i.e. the assumption is your line of code:
User.find_by_import_id(download.user_id)
hits the database to do the lookup. the import_users / import_items tables are just the names i've given to the lookup tables to do this.
UPDATE downloads
SET downloads.user_id = users.user_id
, downloads.item_id = items.items_id
FROM downloads
INNER JOIN import_users ON downloads.user_id = import_users.import_user_id
INNER JOIN import_items ON downloads.item_id = import_items.import_item_id
Either way (lookup is in DB, or it's derived from code), would it not just be easier to insert the information correctly in the first place? this would mean you can't have any FK's on your table since sometimes they point to one table, and others they point to another. seems a bit odd.