Rails：保存记录是否具有唯一值？

Question

I'm probably searching for the wrong thing here, and I'm dumb so let me explain.

I have a rake task that is polling a web service every 10 minutes via cron, the response is JSON.

For each of the items in the response I need to create a record only if a certain column has a unique value, in this case thats a hash.

Here's the meat of the task:

response.parsed_response['items'].each do |item|
  Item.create({
    :item_hash => item['item_hash']
  })
end

The thing is that I only want to do this if the hash value is unique, is this rudimentary behavior or am I missing some obvious method in ActiveRecord?

Answer 1

Use find_or_create

Item.find_or_create_by_item_hash(item['item_hash'])

If the data already exists Item is not created.

If you are processing lot of records I would use the following approach:

all_hashes = response.parsed_response['items'].map{|item| item['item_hash']}
new_hashes = all_hashes - Item.all(:select => :item_hash, 
               :conditions => {:item_hash => all_hashes}).map(&:item_hash)
Item.create(new_hashes.map{|hash| {:item_hash => hash}})

Answer 2

If you want to go the Railsy route, then ActiveRecord validations are the way to go, like Pan Thomakos suggested. There's nothing wrong with using ActiveRecord with non-form data; AR doesn't know (or care) where the data comes from.

There's an SQL route, too: You can pair a unique index with INSERT IGNORE , and your values will only be inserted if they're unique. If you need to know whether the insert succeeded, you can kill the IGNORE and use a rescue block (you might also be able to get the "# rows changed" value back from MySQL, but I'm not sure how to do this in Rails).

Hope this helps!