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Check if a table exists in Rails

I have a rake task that won't work unless a table exists. I'm working with more than 20 engineers on a website so I want to make sure they have migrated the table before they can do a rake task which will populate that respective table.

Does AR have a method such as Table.exists? How can I make sure they have migrated the table successfully?

How to get last N records with activerecord?

With :limit in query, I will get first N records. What is the easiest way to get last N records?

Random record in ActiveRecord

I'm in need of getting a random record from a table via ActiveRecord. I've followed the example from Jamis Buck from 2006.

However, I've also come across another way via a Google search (can't attribute with a link due to new user restrictions):

 rand_id = rand(Model.count)
 rand_record = Model.first(:conditions => ["id >= ?", rand_id])

I'm curious how others on here have done it or if anyone knows what way would be more efficient.

Why all the Active Record hate? [closed]

As I learn more and more about OOP, and start to implement various design patterns, I keep coming back to cases where people are hating on Active Record.

Often, people say that it doesn't scale well (citing Twitter as their prime example) -- but nobody actually explains why it doesn't scale well; and / or how to achieve the pros of AR without the cons (via a similar but different pattern?)

Hopefully this won't turn into a holy war about design patterns -- all I want to know is **specifically** what's wrong with Active Record.

If it doesn't scale well, why not?

What other problems does it have?

delete_all vs destroy_all?

I am looking for the best approach to delete records from a table. For instance, I have a user whose user ID is across many tables. I want to delete this user and every record that has his ID in all tables.

u = User.find_by_name('JohnBoy')
u.usage_indexes.destroy_all
u.sources.destroy_all
u.user_stats.destroy_all
u.delete

This works and removes all references of the user from all tables, but I heard that destroy_all was very process heavy, so I tried delete_all. It only removes the user from his own user table and the id from all the other tables are made null, but leaves the records intact in them. Can someone share what the correct process is for performing a task like this?

I see that destroy_all calls the destroy function on all associated objects but I just want to confirm the correct approach.

When to add what indexes in a table in Rails

I have a question about Rails database.

• Should I add "index" to all the foreign keys like "xxx_id"?
• Should I add "index" to the automatically created "id" column?

• Should I add "index(unique)" to the automatically created "id" column?

• If I add index to two foreign keys at once (add_index (:users, [:category, :state_id]), what happens? How is this different from adding the index for each key?

  class CreateUsers < ActiveRecord::Migration
    def self.up
      create_table :users do |t|
        t.string :name
        t.integer :category_id 
        t.integer :state_id
        t.string :email
        t.boolean :activated
        t.timestamps
      end
    # Do I need this? Is it meaningless to add the index to the primary key?
    # If so, do I need :unique => true ?
    add_index :users, :id 
    # I don't think I need ":unique => true here", right?
    add_index :users, :category_id # Should I need this?
    add_index :users, :state_id # Should I need this?
    # Are the above the same as the following?
    add_index (:users, [:category, :state_id])
    end
  end
  

Great answer so far. Additional question.

• I should add "index with unique" for xxx_id, right?

Determine if ActiveRecord Object is New Or Created

How can I introspect an ActiveRecord object to determine whether it's new or already created?

How can I avoid running ActiveRecord callbacks?

I have some models that have after_save callbacks. Usually that's fine, but in some situations, like when creating development data, I want to save the models without having the callbacks run. Is there a simple way to do that? Something akin to...

Person#save( :run_callbacks => false )

or

Person#save_without_callbacks

I looked in the Rails docs and didn't find anything. However in my experience the Rails docs don't always tell the whole story.

UPDATE

I found a blog post that explains how you can remove callbacks from a model like this:

Foo.after_save.clear

I couldn't find where that method is documented but it seems to work.

ActiveRecord, has_many :through, and Polymorphic Associations

Folks,

Want to make sure I understand this correctly. And please disregard the case for inheritance here (SentientBeing), trying to instead focus on polymorphic models in has_many :through relationships. That said, consider the following...

class Widget < ActiveRecord::Base
  has_many :widget_groupings

  has_many :people, :through => :widget_groupings, :source => :person, :conditions => "widget_groupings.grouper_type = 'Person'"
  has_many :aliens, :through => :widget_groupings, :source => :alien, :conditions => "video_groupings.grouper_type = 'Alien'"
end

class Person < ActiveRecord::Base
  has_many :widget_groupings, :as => grouper
  has_many :widgets, :through => :widget_groupings
end

class Alien < ActiveRecord::Base
  has_many :widget_groupings, :as => grouper
  has_many :widgets, :through => :widget_groupings  
end

class WidgetGrouping < ActiveRecord::Base
  belongs_to :widget
  belongs_to :grouper, :polymorphic => true
end

In a perfect world, I'd like to, given a Widget and a Person, do something like:

widget.people << my_person

However, when I do this, I've noticed the 'type' of the 'grouper' is always null in widget_groupings. However, if I to something like the following:

widget.widget_groupings << WidgetGrouping.new({:widget => self, :person => my_person})

Then all works as I would have normally expected. I don't think I've ever seen this occur with non polymorphic associations and just wanted to know if this was something specific to this use case or if I'm potentially staring at a bug.

Thanks for any help!

Ruby on Rails. How do I use the Active Record .build method in a :belongs to relationship?

I have been unable to find any documentation on the .build method in Rails (i am currently using 2.0.2).

Through experimentation it seems you can use the build method to add a record into a has_many relationship before either record has been saved.

For example:

class Dog < ActiveRecord::Base
  has_many :tags
  belongs_to :person
end

class Person < ActiveRecord::Base
  has_many :dogs
end

# rails c
d = Dog.new
d.tags.build(:number => "123456")
d.save # => true

This will save both the dog and tag with the foreign keys properly. This does not seem to work in a belongs_to relationship.

d = Dog.new
d.person.build # => nil object on nil.build

I have also tried

d = Dog.new
d.person = Person.new
d.save # => true

The foreign key in Dog is not set in this case due to the fact that at the time it is saved, the new person does not have an id because it has not been saved yet.

My questions are:

1. How does build work so that Rails is smart enough to figure out how to save the records in the right order?

2. How can I do the same thing in a belongs_to relationship?

3. Where can I find any documentation on this method?

Thank you

How to express a NOT IN query with ActiveRecord/Rails?

Just to update this since it seems a lot of people come to this, if you are using Rails 4 look at the answers by Trung Lê` and VinniVidiVicci.

Topic.where.not(forum_id:@forums.map(&:id))

Topic.where(published:true).where.not(forum_id:@forums.map(&:id))

I'm hoping there is a easy solution that doesn't involve find_by_sql, if not then I guess that will have to work.

I found this article which references this:

Topic.find(:all, :conditions => { :forum_id => @forums.map(&:id) })

which is the same as

SELECT * FROM topics WHERE forum_id IN (<@forum ids>)

I am wondering if there is a way to do NOT IN with that, like:

SELECT * FROM topics WHERE forum_id NOT IN (<@forum ids>)

Case-insensitive search in Rails model

My product model contains some items

 Product.first
 => #<Product id: 10, name: "Blue jeans" >

I'm now importing some product parameters from another dataset, but there are inconsistencies in the spelling of the names. For instance, in the other dataset, Blue jeans could be spelled Blue Jeans.

I wanted to Product.find_or_create_by_name("Blue Jeans"), but this will create a new product, almost identical to the first. What are my options if I want to find and compare the lowercased name.

Performance issues is not really important here: There are only 100-200 products, and I want to run this as a migration that imports the data.

Any ideas?

Count, size, length...too many choices in Ruby?

I can't seem to find a definitive answer on this and I want to make sure I understand this to the "n'th level" :-)

    a = { "a" => "Hello", "b" => "World" }
    a.count  # 2
    a.size   # 2
    a.length # 2

    a = [ 10, 20 ]
    a.count  # 2
    a.size   # 2
    a.length # 2

So which to use? If I want to know if a has more than one element then it doesn't seem to matter but I want to make sure I understand the real difference. This applies to arrays too. I get the same results.

Also, I realize that count/size/length have different meanings with ActiveRecord. I'm mostly interested in pure Ruby (1.92) right now but if anyone wants to chime in on the difference AR makes that would be appreciated as well.

Thanks!

Rails: Validate uniqueness of multiple columns

Is there a rails-way way to validate that an actual record is unique and not just a column? For example, a friendship model / table should not be able to have multiple identical records like:

user_id: 10 | friend_id: 20
user_id: 10 | friend_id: 20

How to return an empty ActiveRecord relation?

If I have a scope with a lambda and it takes an argument, depending on the value of the argument, I might know that there will not be any matches, but I still want to return a relation, not an empty array:

scope :for_users, lambda { |users| users.any? ? where("user_id IN (?)", users.map(&:id).join(',')) : [] }

What I really want is a "none" method, the opposite of "all", that returns a relation that can still be chained, but results in the query being short-circuited.