创建前检查记录是否存在

Question

I have a subscribe method in my controller

  def subscribe

    @subscription = current_user.subscriptions.create(params[:subscription])
    @subscription.title = @stream.title
    @subscription.save

    redirect_to stream_path(@stream)

  end

Before I set the subscription title to the stream title, how can I check if a subscription already exists with the same title?

Answer 1

Like so :

current_user.subscriptions.where(title: @stream.title).present?

But assuming you only want to save it if it is not included elsewhere you can do this :

if current_user.subscriptions.where(title: @stream.title).blank?
  @subscription.title = @stream.title
  @subscription.save
end

An elegant way to perform this is to add the logic to your controller so that it validates uniqueness ( and also prevents race conditions ) :

class Subscription < ActiveRecord::Base
   validates_uniqueness_of :title, scope: :user
end

And then you can check validation in model.save ( again, assuming you don't want it to save if it shares the same title )

Answer 2

To help you out, the suggestion by @Sergio Tulentsev basically means you create a unique index on a column in your db, like this:

#db/migrate/[timestamp]_new_migration.rb
add_index :table, :column, unique: true

This creates an index on your data table , which basically means you cannot make any duplicate data in it. The bottom line - it means you can only insert the data once.

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The save! method is there to update your record if it exists already. First time I've seen this method, so it's been a good lesson for me. If you're having problems with it, it will probably be that your database will not have the index you need