UC Product Dependencies is an Ubercart module which allows you to restrict products from being purchased based on CCK Node Reference dependencies, without complicated access control schemes.
For each of your product content types, you setup a Node Reference field which will store references to product dependencies. The module can be configured to enforce the location of these dependencies either in a user's cart, or in a user's order history.
For example, Product A can only be purchased if a user has previously purchased a Product B. When editing Product A, Product B is referenced as a dependency. Users can see Product A in the store, but when adding it to the cart, they will see an error message saying that they need to first purchase Product B. You can also configure it so the Product B must simply be in the cart. Or, you can choose both (either in the cart, or previously ordered by that user). The module will enforce these rules, even if the dependency is removed from the cart.
It supports multiple dependencies. If Product A depends on Product B and Product C, then using the "OR" behavior, Product A will be restricted unless either Product B or Product C is in the cart. Using the "AND" behavior, Product A will be restricted unless Product B or Product C are both in the cart.
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