You need dynamic columns in your tables. What do you do?
- Create lots of tables to handle it. Nice, now you’ll need more models and lots of additional sqls. Insertion and selection will be slow as hell.
- Use a noSQL database just for this issue. Good luck.
- Create a serialized column. Nice, insertion will be fine, and reading data from a record too. But, what if you have a condition in your select that includes serialized data? Yeah, regular expressions.
Postgresql 8.4+ (also tested with 9.0) with contrib and Rails 3. (It
might work on 2.3.x with minor patches…)
On Ubuntu, this is easy: sudo apt-get install postgresql-contrib-9.0
On Mac …you are screwed. Use a VM. you should use the binary package kindly provided by EnterpriseDB
Homebrew’s Postgres installation also includes the contrib packages: brew install postgres
The master branch already support a custom serialization coder. If you want to use it just put in your Gemfile:
gem 'activerecord-postgres-hstore', git: 'git://github.com/engageis/activerecord-postgres-hstore.git'
If you install them gem from the master branch you also have to insert a line in each model that uses hstore. Assuming a model called Person, with a data field on it, the code should look like:
class Person < ActiveRecord::Base
serialize :data, ActiveRecord::Coders::Hstore
end
Hstore is a PostgreSQL contrib type, check it out first.
Then, just add this to your Gemfile:
gem 'activerecord-postgres-hstore'
And run your bundler:
bundle install
Make sure that you have the desired database, if not create it as the desired user:
createdb hstorage_dev
Add the parameters to your database.yml (these are system dependant), e.g.:
development:
adapter: postgresql
host: 127.0.0.1
database: hstorage_dev
encoding: unicode
username: postgres
password:
pool: 5
Now you need to create a migration that adds hstore support for your PostgreSQL database:
rails g hstore:setup
Run it:
rake db:migrate
Finally you can create your own tables using hstore type. It’s easy:
rails g model Person name:string data:hstore
rake db:migrate
You’re done. Well, not yet. Don’t forget to add indexes. Like this:
CREATE INDEX people_gist_data ON people USING GIST(data);
or
CREATE INDEX people_gin_data ON people USING GIN(data);
To understand the difference between the two types of indexes take a look at PostgreSQL docs.
Once you have it installed, you just need to learn a little bit of new sqls for selecting stuff (creating and updating is transparent). Find records that contains a key named 'foo’:
Person.where("data ? 'foo'")
Find records where 'foo’ is equal to 'bar’:
Person.where("data -> 'foo' = 'bar'")
This same sql is at least twice as fast (using indexes) if you do it that way:
Person.where("data @> 'foo=>bar'")
Find records where 'foo’ is not equal to 'bar’:
Person.where("data -> 'foo' <> 'bar'")
Find records where 'foo’ is like 'bar’:
Person.where("data -> 'foo' LIKE '%bar%'")
If you need to delete a key in a record, you can do it that way:
person.destroy_key(:data, :foo)
This way you’ll also save the record:
person.destroy_key!(:data, :foo)
The destroy_key method returns 'self’, so you can chain it:
person.destroy_key(:data, :foo).destroy_key(:data, :bar).save
But there is a shortcuts for that:
person.destroy_keys(:data, :foo, :bar)
And finally, if you need to delete keys in many rows, you can:
Person.delete_key(:data, :foo)
and with many keys:
Person.delete_keys(:data, :foo, :bar)
hstore keys and values have to be strings. This means true will become "true" and 42 will become "42" after you save the record. Only nil values are preserved.
It is also confusing when querying:
Person.where("data -> 'foo' = :value", value: true).to_sql
#=> SELECT "people".* FROM "people" WHERE ("data -> 'foo' = 't'") # notice 't'
To avoid the above, make sure all named parameters are strings:
Person.where("data -> 'foo' = :value", value: some_var.to_s)
Have fun.
To have hstore enabled when you load your database schema (as happens in rake db:test:prepare), you'll need to uncomment or add the following line in config/application.rb
config.active_record.schema_format = :sql
This will change your schema dumps from Ruby to SQL. If you're unsure about the implications of this change, we suggest reading this Rails Guide.
You can use issues in github for that. Or else you can reach us at twitter: @dbiazus or @joaomilho
- Fork the project.
- Make your feature addition or bug fix.
- Add tests for it. This is important so I don’t break it in a future version unintentionally.
- Commit, do not mess with rakefile, version, or history. (if you want to have your own version, that is fine but bump version in a commit by itself I can ignore when I pull)
- Send me a pull request. Bonus points for topic branches.
Copyright © 2010 Juan Maiz. See LICENSE for details.