在rails3中创建新记录之前如何检查记录是否存在？

Question

Heres what I'm trying to accomplish:

• I have a tagging system in place.
• Tags are created, when Posts are created (posts has_many :tags, :through => :tag_joins.
• A tag join is automatically created when a post is created with tags).

I want to check if the tag already exists. If it does I want to use the existing tag for the tag_join record, rather than creating a new tag record.

Here is my current code, which isn't working.

class Tag < ActiveRecord :: Base
  belongs_to :user
  belongs_to :tag_join
  belongs_to :post

  before_create :check_exists

  def check_exists
    tag = Tag.where(:name => self.name, :user_id => current_user.id)
    if tag.nil?
      tag = Tag.create(:name => self.name, :user_id => current_user.id)
    end
  end

end

This doesn't work though, I'm getting an error upon task creation...(the server is actually just timing out - I don't receive a specific error).

Any ideas?

Tokland said I was creating an infinite loop by telling it to create tag again - so I tried this:

 def check_exists
      tag = Tag.find_by_name_and_user_id(:name => self.name, :user_id => current_user.id)
      if tag != nil
        self.id = tag.id
      end
  end

And still get the server timeout

Edit: I'm not sure if this matters, but the way the tags are being added is similar to "http://railscasts.com/episodes/73-complex-forms-part-1

they're nested in the post form, and use something like this:

def tag_attributes=(tag_attributes)
  tag_attributes.each do |attributes|
    tags.build(attributes)
  end
end

I'm wondering if this is stopping this whole thing from working? Also, using current_user.id in the model definitely seems to be an issue...

EDIT:

Something I have figured out: this had to change, the format we were using before was incorrect syntax - generally used for a .where method.

  def check_exists
     @tag = Tag.find_by_name_and_user_id(self.name, self.user_id) 
     if @tag != nil
       #return false
       #self=@tag
     end
  end

The problem now is this, I can learn if it the tag already exists. But then what? If I go with the return false option, there is an error upon post creation, and the join record isn't created... The other option "self=@tag" obviously just doesn't work.

Answer 1

You're going to find it hard to to this from within the Tag model. It seems like what you want is to update the Post using nested attributes, like so:

post = Post.create
post.update_attributes(:tags_attributes=>{"0"=>{:name=>"fish",:user_id=>"37"}})

This is actually pretty simple to do by using a virtual attribute setter method:

class Post < AR::Base
  has_many :tags

  def tags_attributes=(hash)
    hash.each do |sequence,tag_values|
      tags <<  Tag.find_or_create_by_name_and_user_id(tag_values[:name],\
        tag_values[:user_id])
    end
  end

> post = Post.create
> post.update_attributes(:tags_attributes=>{"0"=>{:name=>"fish",:user_id=>"37"}})
> Tag.count # => 1
# updating again does not add dups
> post.update_attributes(:tags_attributes=>{"0"=>{:name=>"fish",:user_id=>"37"}})
> Tag.count # => 1

Answer 2

There's a find_or_create_by_ function built right in to Rails

# No 'Summer' tag exists
Tag.find_or_create_by_name("Summer") # equal to Tag.create(:name => "Summer")

# Now the 'Summer' tag does exist
Tag.find_or_create_by_name("Summer") # equal to Tag.find_by_name("Summer")

http://api.rubyonrails.org/classes/ActiveRecord/Base.html (under Dynamic attribute-based finders)

Answer 3

You want to use the magic method find_or_create_by

def check_exists
    tag = Tag.find_or_create_by_name_and_user_id(:name => self.name, :user_id => current_user.id)
end

Check out the ActiveRecord::Base docs for more info

Answer 4

The question I originally asked got pretty distorted by the end. So I'm separating it.

People who are trying to do what I originally asked can try this:

 before_create :check_tag_exists

 private

 def check_tag_exists
     @tag = Tag.find_by_name_and_user_id(self.name, self.user_id)
     if @tag != nil
       #
     end
  end

This will enable you to check if your record has already been created. Any further logic you can drop in that if statment.

Answer 5

I believe the other answers are a bit dated. Here's how you should probably accomplish this for Rails 4

tag = Tag.first_or_initialize(:name => self.name, :user_id => current_user.id)
if !tag.new_record?
    tag.id = self.id
    tag.save
end

Answer 6

try this

  def check_exists
    tag = Tag.where(:name => self.name, :user_id => current_user.id).first
    tag = Tag.new({:name => self.name, :user_id => current_user.id}) unless tag
  end

use Tag.new instead of Tag.create

Answer 7

where 在找不到匹配项时返回一个空的 ActiveRecord。