eav_hashes

eav_hashes is a neato gem for implementing the EAV (entity-attribute-value) database design pattern in your Rails models. All you need to do is add one line to your model's code and that's it! Schema generation is automatically handled for you.

Rails' ActiveRecord includes a helper function, serialize, to allow you to save complex data types (like hashes) into your database. Unfortunately, it isn't very useful. A lot of overhead is created from serialization and deserialization, and you can't search by the contents of your hash. That's where eav_hashes comes in.

Great question! Lets dive in with a simple code example, but first lets set up the gem.

Put this in your gemfile...

gem "eav_hashes", "~> 1.1.0"

...and update your bundle.

$ bundle install

Now, lets make this Rails model.

class Product < ActiveRecord::Base
    eav_hash_for :tech_specs
end

Now run this generator to create a migration:

$ rails generate eav_migration Product tech_specs

And run the migration:

$ rake db:migrate

Now watch the magic the happen:

# Assuming this whole example is on a blank DB, of course
a_product = Product.new
a_product.tech_specs["Widget Power"] = "1.21 GW"
a_product.tech_specs["Battery Life (hours)"] = 12
a_product.tech_specs["Warranty (years)"] = 3.5
a_product.tech_specs["RoHS Compliant"] = true
a_product.save!

# Setting a value to nil deletes the entry
a_product.tech_specs["Warranty (years)"] = nil
a_product.save!

the_same_product = Product.first
puts the_same_product.tech_specs["nonexistant key"]

# magic alert: this actually gets the count of EVERY entry of every
# hash for this model, but for this example this works
puts "Entry Count: #{ProductTechSpecsEntry.count}"
the_same_product.tech_specs.each_pair do |key, value|
    puts "#{key}: #{value.to_s}"
end

# Ruby's default types: Integer, Float, Complex, Rational, Symbol,
# TrueClass, and FalseClass are preserved between transactions like
# you would expect them to.
puts the_same_product.tech_specs["Battery Life (hours)"]+3

And the output, as you can expect, will be along the lines of:

nil
Entry Count: 3
Widget Power: 1.21 GW
Battery Life (hours): 12
RoHS Compliant: true
15

That looks incredibly simple, right? Good! It's supposed to be! All the magic happens when you call save!.

Now you could start doing other cool stuff, like searching for products based on their tech specs! You've already figured out how to do this, haven't you?

flux_capacitor = Product.find_by_tech_specs("Widget Power", "1.21 GW")

Nifty, right?

Sure, but they'll be serialized into YAML (so you cant search by them like you would an eav_hash). The value column is a TEXT type by default but if you want to optimize your DB size you could change it to a VARCHAR in the migration if you don't plan on storing large values.

By default, eav_hash uses a table name derived from the following:

"<ClassName>_<hash_name>".tableize

You can change this by passing a symbol to the :table_name argument:

class Widget < ActiveRecord::Base
    eav_hash_for :foobar, table_name: :bar_foo
end

Just remember to edit the table name in the migration, or use the following migration generator:

$ rails generate eav_migration Widget foobar bar_foo

By using this software, you agree to write me into your will as your next of kin, and to sacrifice the soul of your first born child to Beelzebub.

Just kidding, the code is released under the MIT license so you can use it for whatever purposes you see fit. Just don't sue me if your application blows up from the sheer awesomeness! Check out the LICENSE file for more information.

Thanks to Matt Kimmel for adding support for models contained in namespaces.

Thanks to Arpad Lukacs for adding Enumerator-like behavior.

Thanks to Christopher Bartling and jenskdsgn for fixing deprecation warnings in Rails 5.

You're probably reading this from GitHub, so you know what to do. If not, the Github project is at https://github.com/iostat/eav_hashes