Re: [Rails] Dynamically create databases?

Ruby on Rails Thursday, April 4, 2013

Okay...


So to do this, you need to understand meta-programming to a degree because that's what you're doing.

In normal Rails, you'd create a model as part of the development process, but what you're doing is creating some code that creates models itself (ie one level of abstraction higher than normal).

The equivalent straight ruby comparison is: (compare 1, with 2, below):

1. Creating a class (this is normal programming)

class Cat
  def hi
    puts 'meow'
  end
end

You can then create new Cat objects like this:
furrycat = Cat.new

and make it meow like this:
furrycat.hi

2. Creating a piece of code that creates a class (this is meta-programming)

a_class = Class.new
hi_method_block = Proc.new{ puts 'meow' }
a_class.send(:define_method, :hi, &hi_method_block)

You can then create new "Cat" objects like this:
furrycat = a_class.new

and make it meow like this:
furrycat.hi

---

So...  assuming you followed that, then you'll understand that you could do the same thing with ActiveRecord::Base classes, which is what forms the bases for Active Record model classes. This is how you'd dynamically LOAD one of your programmatically generated models. Creating the table is just a matter of executing some arbitrary SQL which can be built using ActiveRecord::Base.connection.execute. (Eg to run a count of a fictitious users table, you'd do this: ActiveRecord::Base.connection.execute("SELECT COUNT(*) FROM users;")

However, like I said, there's no point trying to do this until you can walk, because this really is running. Most Rails devs hardly ever get into this stuff. I've been a Rails developer since 2005, and I've only done this sort of dynamic table and database thing once or twice in practice.

Julian

On 04/04/2013, at 6:43 PM, Johan Vauhkonen <johan.vauhkonen@gmail.com> wrote:

Thanks for posting, I appreciate the feedback.

I'll start with keeping everything within a single database and take it from there.

You are right Julian in that I am new with RoR and what I've asked for is advanced.

I'm still curious though how creating databases dynamically would be done so if it's explained anywhere I'd love to read it!


2013/4/4 Julian Leviston <julian@coretech.net.au>

On 04/04/2013, at 6:05 PM, Johan Vauhkonen <johan.vauhkonen@gmail.com> wrote:

> Thanks for replying, Julian.
>
> Can you point me to any resources that describe how to do it?
>
> I agree that I should not optimize prematurely but what I'm considering is which is easier,
> to go with dynamically creating databases from the start or to extract data from the single database to new databases later on?
>
> It's at least something to keep in the back of my mind.

Hi,

You shouldn't attempt to do this if you don't already understand enough Ruby / Rails to do it yourself.

So, I suggest sticking with what you *can* do first.

This might sound like a cop out, but there's very good reason. It's very advanced Rails and you really shouldn't attempt something like this until you understand the basics really well IMHO.

Julian

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Posted by Ruby on Rails at 1:34 AM

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