This is most likely a noob question since people use this gem and a lot of people love it, but I don't get the purpose. I'm looking at a project and its been used here many times in places such as t.references :foreign_key_table_name , :foreign_key => true, add_foreign_key :table :foreign_key_table_name, :options, and in a create t.foreign_key :foreign_key_table_name. Hope those weren't confusing since they're out of context.
But I don't get how this is different from what rails does built in with t.references :foreign_key_table_name or from me just adding t.integer :foreign_key_table_name_id? does it simply make it more readable by making clear that this is a 'foreign key'? I could just add a comment instead of a gem if thats the case... The only advantage I see is that you can move options such as :dependent into the migration instead of having it in the model, but who cares?
Some database engines support legit foreign key constraints: if someone tries to save a Child with a parent_id of 5, but there's no Parent with id 5, then the database itself (not Rails) will reject the record if there's a foreign key constraint linking children.parent_id and parents.id.
A foreign key can also specify what happens if the parent is deleted: in MySQL, for example, we can delete or nullify the dependent records, like how Rails does with :dependent, or even just straight-up reject the deletion and throw an error instead.
Since not all database engines offer this functionality, Rails offers to emulate it with :dependent, and it's nice to have it on the software level so that dependent child records can fire their destroy callbacks when the parent is deleted. Since the feature is engine-independent and therefore pretty much schema-independent, Rails doesn't handle the creation/deletion of foreign keys. That's where foreigner comes in: if your engine supports foreign key constraints, and you want that extra confident in your data integrity, foreigner can help with that.
Resurrecting an old question here, but…
Having rails enforce the relationship is fine, within rails itself.
However, if your project grows to have code that also accesses these tables from other languages, that will not have the benefit of rails enforcing the relations. These foreign key constraints are baked into the SQL tables themselves, so can protect non-rails code.
This will also protect you if you need to perform datafixes or otherwise manipulate your data via native SQL.
Another reason is that some documentation tools for SQL look at foreign keys on the DB, so it is cool to have a gem that generates them. Rails 4 added the ability to define foreign keys in the same migration that creates the table with:
t.references :something, foreign_key: true
And the generators will do this for you if you use the references type. Rails adds an index on something_id by default when using foreign_key like this
Related
By release of rails 4.2 add_foreign_key method was introduced. As far as I understand it is used as combo of migration with model:references and add_index.
Let's say I only use PostreSQL. (add_foreign_key is limited to MySQL and PostreSQL). Should stop using migration model:references with add_index and start using add_foreign_key only?. If yes/no, why?. What are benefits of new add_foreign_key method? Is it worth to swap?
Foreign key constraints can help with referential integrity (you can't insert data belonging to a book that doesn't exist for example). Foreign keys also provide database level referential integrity as opposed to application level (model validation) integrity.
The Rails team felt it important enough that they now automatically create foreign keys whenever you use references in generating your migrations.
I'm just playing with RoR and I've noticed that ActiveRecord associations such as has_many or belongs_to are decoupled from the database running behind, i.e., these association are set regardless of the the constraints set by the database. For example, I have a table comments and a table users and they are related through has_many and belongs_to statements (a comment belongs to a user and a user has many comments). However these associations still let me assign a comment to a, for example, non-existing user. The reason of this is that there's no foreign key defined in the database.
My question is: should I just rely on ActiveRecord's associations to handle data integrity or should I also add foreign keys in migration files?
Thank you.
Rails holds some conventions that enforcement of data integrity should be done in the application, not in the database.
To keep data integer on application-level, you can use model validations to enforce the presence of associations.
You have to add foreign keys to migration file to make your associations work correctly. With reference to the example mentioned, you have to add an attribute user_id to comments table. For more information on how Active Record Associations work, follow this rails guide.
I am a beginner to rails framework. I have a fundamental question.
I am trying to define some models and their association referring the popular rails guides. My association looks like below.
class Person < ActiveRecord::Base
has_one :head
end
class Head < ActiveRecord::Base
belongs_to :person
end
Here, I need to add the foreign_key (Persons's primary key) in the table 'head'.
Now, if I need to get the 'head' of a 'person', rails need to scan through the head table and match the person_id.
The straight forward way I would think is to add the foreign key in 'person' table. Then I can directly refer the 'head' from 'person' with it's ID.
It appears that rails convention is not performance friendly. Am I missing something here?
When you create the migration to add the column containing the foreign key, it is highly recommended to add an index on this column. This way, the database will efficiently find the Head from the person_id (as efficiently then a search by its id).
add_index :heads, :person_id
If it's a one-to-one association, you can even add the unique option (unless your application accepts conjoined twins :-) ):
add_index :heads, :person_id, :unique => true
I suggest you to have a look to this 2 articles on where to use indexes:
http://tomafro.net/2009/08/using-indexes-in-rails-index-your-associations
http://tomafro.net/2009/08/using-indexes-in-rails-choosing-additional-indexes
Re: It appears that rails convention is not performance friendly. Am I missing something here?
Yes. Rails is designed for production sites. So it includes many performance features. As #Baldrick says, the answer to your specific concern is to always add an index for foreign key fields.
Adding an index for each foreign key field is needed, for performance reasons, for any SQL dbms application, no matter the language. Note that the index is added to the database, not to the MVC layers (Rails).
Rails itself includes additional performance features including sql results caching, optional fragment and page caching, and more.
Rails 3.2 includes the slow query features. These enable Rails to automatically show you the queries which are slow. You can then focus on fixing them as appropriate.
I think there are a lot of places where my design may be screwing this up. I have very limited experience with Rails though. This is happening in Rails 2.3.2 with Postgres 8.3.
We've got two tables in our DB. One called "survey" and one called "survey_timepoint". A survey can have multiple time points so in the survey_timepoint table there is a column called "survey_id" with an fk constraint on it.
I also think I should mention that the tables were not created with a rails migration although they do follow the rails naming conventions. I suspect AR isn't anticipating a constraint on that column and it doesn't know how to handle the situation.
In my rails models I have:
has_many :survey_timepoint
and
belongs_to :survey
If I do something like:
s = Survey.new
s.survey_timepoint.push SurveyTimepoint.new
s.save!
I get:
ActiveRecord::StatementInvalid: PGError: ERROR: insert or update on table "survey_timepoints" violates foreign key constraint "survey_timepoints_fk"
DETAIL: Key (survey_id)=(59) is not present in table "surveys"
I'm assuming that if I delete that fk constraint on survey_timepoint.survey_id it'll work ok. It seems like I shouldn't have too though. Am I going to be stuck creating and saving each of the objects separately and wrapping the whole process in a transaction? It seems rather un-railsy. Apologies for any necessary information that I may have omitted.
You might want to check the SQL commands being sent. It looks like it is adding the survey_timepoint record before the survey record. Note that you are already dealing with two database changes — the survey and the survey_timepoint — so you should be using a transaction.
You can fix the immediate problem by doing s.save! before adding the timepoint (and then calling it again). My knowledge of Rails functionality is not deep enough to know if there is a more "railsy" way of doing this then wrapping it in a transaction.
I just experimented and found that this works with MySQL:
s = Survey.new()
s.survey_timepoints << SurveyTimepoint.new # Note "survey_timepoints" (plural)
s.save!
I think it would work equally well with PostgreSQL.
It does two inserts, first the Survey, then the timepoint, and wraps them in a transaction.
You can also do it all on one line:
Survey.create!({:name=>'New Survey', :survey_timepoints => [SurveyTimepoint.new]})
Incidentally, for ActiveRecord to work right you have to make sure of your singulars and plurals. (If you want to break the expected forms, you'll need to tell AR you're doing that -- a whole other topic.)
Your tables should be:
surveys
-------
# ...
survey_timepoints
-----------------
survey_id
# ...
And in your models you'd have:
class Survey < ActiveRecord::Base
has_many :survey_timepoints
# etc...
end
class SurveyTimepoint < ActiveRecord::Base
belongs_to :survey
end
I don't have a Rails environment set up and this is actually quite hard to find a quick answer for, so I'll ask the experts.
When Rails creates a table based on your "model" that you have set up, does Rails create a table that mirrors this model exactly, or does it add in more fields to the table to help it work its magic? If so, what other fields does it add and why? Perhaps you could cut and paste the table structure, or simply point me to a doc or tutorial section that addresses this.
If you're building a completely new application, including a new database, then you can build the whole back end with migrations. Running
ruby script/generate model User name:string
produces both a user.rb file for the model and a migration:
class CreateUsers < ActiveRecord::Migration
def self.up
create_table :users do |t|
t.string :name
t.timestamps
end
end
def self.down
drop_table :users
end
end
You can see that by default the generate script adds "timestamps" for (created and last updated) and they're managed automatically if allowed to remain present.
Not visible, but important, is that an extra column, "id", is created to be the single primary key. It's not complusory, though - you can specify your own primary key in the model, which is useful if you're working with a legacy schema. Assuming you retain id as the key, then Rails will use whatever RDBMS-specific features are available for new key values.
In ActiveRecord, models are created from database tables, not the other way around.
You may also want to look into Migrations, which is a way of describing and creating the database from Ruby code. However, the migration is not related to the model; the model is still created at runtime based on the shape of the database.
There are screencasts related to ActiveRecord and Migrations on the Rails site: http://www.rubyonrails.org/screencasts
Here's the official documentation for ActiveRecord. It agrees with Brad. You might have seen either a different access method or a migration (which alters the tables and thus the model)
I have had a little experience moving legacy databases into Rails and accessing Rails databases from outside scripts. That sounds like what you're trying to do. My experience is in Rails databases built on top of MySQL, so your mileage may vary.
The one hidden field is the most obvious --- the "id" field (an integer) that Rails uses as its default primary key. Unless you specify otherwise, each model in Rails has an "id" field that is a unique, incremented integer primary key. This "id" field will appear automatically in any model generated within Rails through a migration, unless you tell Rails not to do so (by specifying a different field to be the primary key). If you work with Rails databases from outside Rails itself, you should be careful about this value.
The "id" field is a key part of the Rails magic because it is used to define Rails' associations. Say you relate two tables together --- Group and Person. The Group model will have an "id" field, and the Person model should have both its own "id" field and a "group_id" field for the relationship. The value in "group_id" will refer back to the unique id of the associated Group. If you have built your models in a way that follows those conventions of Rails, you can take advantage of Rails' associations by saying that the Group model "has_many :people" and that the Person model "belongs_to :group".
Rails migrations also, by default, want to add "created_at" and "updated_at" fields (the so-called "timestamps"), which are datetime fields. By default, these take advantage of the "magic" in the database --- not in Rails itself --- to automatically update whenever a record is created or modified. I don't think these columns will trip you up, because they should be taken care of at the database level, not by any special Rails magic.