an advice on how to use associations.

I have these models:

Company;
Categories;
Classifications.

A Company has many categories and a Category has many companies.
A Company for each of its own categories has a Classification.
For example:
Company-1 has Category-1 and Category-2.
Company-1 - Category-1 has Classification-III while Company-1 -
Category-2 has Classification IV.
How can I declare associations?

Relevant documentation is here:

http://guides.rubyonrails.org/association_basics.html

(For similar results, google for "Rails Guides ").

If I understand correctly, your case would be a perfect case for

has_many :through (and not habtm) because you need

Classification attributes on each association between a

Company and a Category (then the Classification table is

the association table between Company and Category).

Try to work out the code for your case from that documentation.

If you have difficulties with it, please come back with your specific

problems, showing us the code your wrote.

HTH,

Peter

I'm also was thinking about has_many :through.
The join model is Classification with attriìbutes like:
company_id
category_id
classification_type
classification_amount.

But if classification.amount for classification.type II change I must
change all the occurrences for company and categories where
classification.type II occurs.

I think its better to use a join table like this:
company_id
category_id
classification_id

What do you think about?

Indeed. I had not understood there are only a limited number of
Classification types possible between the Company and the
Category.

In that case, indeed it is better that the association table does

not have the Classification attributes itself, but you normalized
out these attributes to a third Classification table (with the
Classification#classification_type Classification.amount attributes).

WARNING: do not use ‘type’ as a column name, it is a special

column that is used for STI (Single Table Inheritance). Unless
you would really want to use STI on your Classifications (which
might make sense in an OO design if certain Classification types
have special functionality that is different from other Classification

types).

HTH,

Peter