# Any ideas on how to model this? **URL:** \ **Category:** Rails\ **Created:** [November 15, 2006, 4:01am UTC](https://www.ruby-forum.com/t/any-ideas-on-how-to-model-this/76009 "2006-11-15T04:01:49Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![runrunshaw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/b5ac83/32.png) [@runrunshaw](https://www.ruby-forum.com/u/runrunshaw)\ **Post date:** [November 15, 2006, 4:01am UTC](https://www.ruby-forum.com/t/any-ideas-on-how-to-model-this/76009/1 "2006-11-15T04:01:49Z")
Hello, I’m attempting to move beyond the Agile Web Dev w\Rails book and explore writing some practice apps from scratch. I m attempting to model an online t-shirt shop were customers can select from a variety of different designs. Each design will be available in different colors and each color will have different sizes. Also, I’d like to keep track of the number of available shirts. My most recent data modeling attempt consists of 3 tables and respective columns: 1. designs - name, description, picture, price. 2. color - color, design\_id 3. size - size, color\_id The model associations are: design - has\_many :colors, has\_many :sizes color - has\_many :sizes, belongs\_to :designs size - belongs\_to :color, :belongs\_to :design SO far, so good! The problem is having to keep an inventory of the number of available shirts (i.e. quantity). I have a 4th table called ‘quantity’ which has the following columns: quantity, design\_id, color\_id, size\_id since it isn’t strictly associated with a single design, color or size but all three. But… From here I’m lost. I have tried using a HABTM association between the design table and quantity table but something just didn’t seem right. So I’m asking for help. What is the best way to model this? Does the ‘quantity’ table need to be on it’s own or can I perhaps rework it into the other 3 tables? Any help is greatly appreciated!!! ---
**Author:** ![runrunshaw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/b5ac83/32.png) [@runrunshaw](https://www.ruby-forum.com/u/runrunshaw)\ **Post date:** [November 15, 2006, 4:22am UTC](https://www.ruby-forum.com/t/any-ideas-on-how-to-model-this/76009/2 "2006-11-15T04:22:50Z")
Hi Richard, Richard D. wrote: > I m attempting to model an online t-shirt shop > > My most recent data modeling attempt consists > of 3 tables and respective columns: > > 1. designs - name, description, picture, price. > 2. color - color, design\_id > 3. size - size, color\_id > > The model associations are: … largely irrelevant until you have all the models you need. If you’re modeling a t-shirt shop, one might expect a t-shirt model. That might have a quantity-on-hand attribute. Of course, that’s just one way to look at it. hth, Bill ---
**Author:** ![runrunshaw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/b5ac83/32.png) [@runrunshaw](https://www.ruby-forum.com/u/runrunshaw)\ **Post date:** [November 15, 2006, 11:13pm UTC](https://www.ruby-forum.com/t/any-ideas-on-how-to-model-this/76009/3 "2006-11-15T23:13:59Z")
On 11/15/06, Richard D. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > My most recent data modeling attempt consists of 3 tables and respective > columns: > > 1. designs - name, description, picture, price. > 2. color - color, design\_id > 3. size - size, color\_id > > The model associations are: > design - has\_many :colors, has\_many :sizes > color - has\_many :sizes, belongs\_to :designs > size - belongs\_to :color, :belongs\_to :design The relationships are not right. You can break them easily by creating a color belonging\_to one design, and a size belonging to the same color, but a different design. If you drew some sort of model diagram, you would see that you have created a triangle between the three entities, which is (almost) always a Very Bad Idea. You would also have to model the same color (e.g. Blue) multiple times - one for each design; and model each size (S/M/L) once for each color/design combination. You have a need for an explicit join model. There are a few different ways of modelling this, but let’s introduce a new model class calle Variation - this is a fairly common term in this kind of situation. Normally, each Variation of a product in a shop will have it’s own unique product number of some sort. Your model might look like this: Variation belongs\_to :design belongs\_to :color belongs\_to :size Design has\_many :variations has\_many :colors, :through=\>:variations has\_many :sizes, :through=\>:variations This way, while logically your Design model offers the same access to the colors and sizes, you have created a _derived_ relationship between the classes, which is much preferrable to the Triangle Of Doom. To get a list of colors for a chosen design is simple: some\_design.colors To get a list of all variations for a design and a color (from which you can get the sizes): some\_design.variations.find\_by\_color\_id(the\_color\_id) Or get the sizes directly: some\_design.variations.find\_by\_color\_id(the\_color\_id).collect { |v| v.size } The last two should probably be implemented as methods on the Design class (get\_variations\_for\_color(color), get\_sizes\_for\_color(color)) You could then ad the stock quantities to the Variaion model, or model them in a separate table with a has\_one relationship. Cheers, Max ---
**Author:** ![runrunshaw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/b5ac83/32.png) [@runrunshaw](https://www.ruby-forum.com/u/runrunshaw)\ **Post date:** [November 16, 2006, 12:52am UTC](https://www.ruby-forum.com/t/any-ideas-on-how-to-model-this/76009/4 "2006-11-16T00:52:15Z")
Awesome info Max! I’m working on modeling a scenario of my own and although it’s a little bit different from this one, reading your post on how to tackle this problem was VERY insightful. ---
**Author:** ![runrunshaw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/b5ac83/32.png) [@runrunshaw](https://www.ruby-forum.com/u/runrunshaw)\ **Post date:** [November 17, 2006, 3:49am UTC](https://www.ruby-forum.com/t/any-ideas-on-how-to-model-this/76009/5 "2006-11-17T03:49:54Z")
Thank you very much. This is just what I was looking for!!!