cartmagento-1.9price
I have a specific shopping cart rule that checks weather 3 items or greater of products from certain categories have been added to the cart, and then gives a discount on a specific Item ( I will call it "Y" in this case).
Magento will only apply the discount on Item Y if it has been added to the cart.
What I am trying to do is display the option to add the product "Y" on the cart page (cart.phtml) if the user has not yet added it to the basket.
There aren't many ways to promote product diversity - they have to be set up for specific category pairings. If you have a limited number of categories you may be able to set up enough combinations to apply to a good portion of your catalog.
See this example:
Set up multiple rules
It will look something like this:
The individual sales rules are possible, so you can create a cart rule that will give a percentage for any order containing items bought from a given category. You can then create other rules for other categories. So each of them could cumulatively give e.g. 2% off the order.
However, those rules are not aware of each other. So they do not know what the current discount is (from the other rules) and cannot then have the 'if' to decide whether a further discount is possible.
As for the product specific discount, you can do that one with a rule or a just a special price on the product.
If you want to do this with a module then the easy bit is coding the rules that you want. Quite a few other changes are needed. There may be a need to update templates (to show your extra line item), some xml for 'totals_sort' order - it goes on.
I also feel that your original sales idea might actually be confusing for customers. Sometimes the spend X get Y rules need to be easy for people to understand.
Best Answer
Welcome to the rabbit hole. You are going to have to deal with Mage/SalesRule and most likely the generic Mage/Rule.
However, as I've mentioned before, shopping cart rule is a very bad name. They're not shopping cart rules, they are checkout, aka Cash Register rules, as they're not evaluated in the cart but in the checkout.
If you really want to do this, you will have to duplicate the rule evaluation code from the checkout and probably alter it, because some rules (like free shipping) depend on the availability of an address object or customer group, which you can't count on. Start with Mage::getModel('salesrule/rule')->getCollection().
Personally, I would hardcode this and just check for the products in question in the cart, if this is a one time deal, but if this is common functionality for the shop, I wish you the best of luck ;-)
Edit: Assuming you have at least a rule ID, you can get conditions using
Mage::getModel('salesrule/rule')->load($id)->getConditions(). The implementation is inMage_Rule_Model_Abstract.