在保存到数据库之前检查记录是否存在-Rails，Twitter，ActiveRecord

Question

I am using a Twitter Gem to pull and save Tweets from user's timeline. Before saving each message , I want to check if this record already exists by comparing the new message's tweet_id to the array of tweet_id 's already saved in the db. No matter what I try, I am still seeing duplicated records being saved in the messages table. Here is what I got:

Controller call:

@messages = Message.pull_tweets(@user)

"pull_tweets" method:

def self.pull_tweets(user)

    tweets = $twitter.home_timeline  # API call to pull all tweets from the timeline

    if tweets && tweets.each { |tweet| validate_uniqueness_of(user,tweet) }
      tweets.each { |tweet| user.messages.create!(tweet_id: tweet.id ... }
      ...
    else
      ... 
    end
  end

"validate_uniqueness_of" method:

  def self.validate_uniqueness_of(user,tweet)
    if user.messages.pluck(:tweet_id).exclude?(tweet.id.to_s)
      return true
    else
      return false
    end
  end

Answer 1

The most direct cause of your problem is that tweets.each will return the tweets array, which since it is not nil or false has a truthy value: you're not using the result of your validate_uniqueness_of method.

You instead would want to either do something like

tweets.all? { |tweet| validate_uniqueness_of(user,tweet) }

which returns true only if all the tweets meet your test, or more likey to be what you want

if tweets
  tweets = tweets.select { |tweet| validate_uniqueness_of(user,tweet) }
  tweets.each { |tweet| user.messages.create!(tweet_id: tweet.id ... } 
end 

This would however be rather non idiomatic code. You would usually create a validation on your tweet class and add a unique index on the tweet_id column - a uniqueness validation should always be backed up by a unique index or you run the risk of rice conditions.

The validation side of things would look like

class Message < ActiveRecord::Base
  validate_uniqueness_of :tweet_id, scope: :user_id #assuming that message belongs_to :user
end

You can either keep using create! and rescue the validation exception that will be thrown or switch to create which returns true/false depending on what happened. In both cases ActiveRecord::RecordNotUnique will be raised if a duplicate slips past the validation

Answer 2

您还可以根据：scope参数来验证tweet_id是否唯一：

validates_uniqueness_of :tweet_id, scope: :user_id