loopback relational database hasManyThrough pivot table - node.js

Question

I seem to be stuck on a classic ORM issue and don't know really how to handle it, so at this point any help is welcome.
Is there a way to get the pivot table on a hasManyThrough query? Better yet, apply some filter or sort to it. A typical example
Table products
id,title
Table categories
id,title
table products_categories
productsId, categoriesId, orderBy, main
So, in the above scenario, say you want to get all categories of product X that are (main = true) or you want to sort the the product categories by orderBy.
What happens now is a first SELECT on products to get the product data, a second SELECT on products_categories to get the categoriesId and a final SELECT on categories to get the actual categories. Ideally, filters and sort should be applied to the 2nd SELECT like
SELECT `id`,`productsId`,`categoriesId`,`orderBy`,`main` FROM `products_categories` WHERE `productsId` IN (180) WHERE main = 1 ORDER BY `orderBy` DESC
Another typical example would be wanting to order the product images based on the order the user wants them to
so you would have a products_images table
id,image,productsID,orderBy
and you would want to
SELECT from products_images WHERE productsId In (180) ORDER BY orderBy ASC
Is that even possible?
EDIT : Here is the relationship needed for an intermediate table to get what I need based on my schema.
Products.hasMany(Images,
{
as: "Images",
"foreignKey": "productsId",
"through": ProductsImagesItems,
scope: function (inst, filter) {
return {active: 1};
}
});
Thing is the scope function is giving me access to the final result and not to the intermediate table.

Answer 1

I am not sure to fully understand your problem(s), but for sure you need to move away from the table concept and express your problem in terms of Models and Relations.
The way I see it, you have two models Product(properties: title) and Category (properties: main).
Then, you can have relations between the two, potentially
Product belongsTo Category
Category hasMany Product
This means a product will belong to a single category, while a category may contain many products. There are other relations available
Then, using the generated REST API, you can filter GET requests to get items in function of their properties (like main in your case), or use custom GET requests (automatically generated when you add relations) to get for instance all products belonging to a specific category.
Does this helps ?

Answer 2

Based on what you have here I'd probably recommend using the scope option when defining the relationship. The LoopBack docs show a very similar example of the "product - category" scenario:
Product.hasMany(Category, {
as: 'categories',
scope: function(instance, filter) {
return { type: instance.type };
}
});
In the example above, instance is a category that is being matched, and each product would have a new categories property that would contain the matching Category entities for that Product. Note that this does not follow your exact data scheme, so you may need to play around with it. Also, I think your API query would have to specify that you want the categories related data loaded (those are not included by default):
/api/Products/13?filter{"include":["categories"]}

Answer 3

I suggest you define a custom / remote method in Product.js that does the work for you.
Product.getCategories(_productId){
// if you are taking product title as param instead of _productId,
// you will first need to find product ID
// then execute a find query on products_categories with
// 1. where filter to get only main categoris and productId = _productId
// 2. include filter to include product and category objects
// 3. orderBy filter to sort items based on orderBy column
// now you will get an array of products_categories.
// Each item / object in the array will have nested objects of Product and Category.
}