Strategy: catalog connection with managed metadata multivalue field

Everybody knows about SharePoint catalog connection... maybe

What if you have a pre-existing catalog connection and your customer asks you to switch from a single value field (generally ItemCategory) to a multivalue field?

In a ideal world you should quote this and say "don't do it, don't even think about it"

In a business world you'll say "Yes, why not?".

I tried to convert the single value field in a multivalue field... trust me... don't try this if you don't like strange side effects.

AS IS

This is the typical product catalog configuration:

• A product catalog site which is used as backend website
• Contents are crawled by search service application
• Public sites connect to indexed catalog

Solution
To understand next steps, mind every link under categories navigation term is allowed.
Let's assume you have a term set like that

• Category 1
• Subcategory 11
• Subcategory 12
• Category 2 
• Subcategory 21

The catalog connection automatically creates these category friendly urls:

http://site/category1
http://site/category1/subcategory11
http://site/category1/subcategory11
http://site/category2
http://site/category2/subcategory21

So let's assume i have a product named Test, which is tagged on ItemCategory with term "Subcategory 11", i'll go to URL http://site/category1/subcategory11/test .

But what if i tell you that links like these below will not repond with a 404?
http://site/category1/test
http://site/category1/subcategory12/test
http://site/category2/test
http://site/category2/subcategory21/test

This behaviour will be extremely useful for us.

Editing product catalog
Because you don't want to destroy catalog connection, you just have to add a TaxonomyFieldTypeMulti field to the product catalog item content type

  
    
   
     TextField
     {024FECDC-E8A7-4DAC-BEB1-E9C373708BE5}
   
    
  

Link this new field to the term set you use to tag catalog items.
After that, you can create a event receiver in order to write the first  value entered in MultivalueItemCategory field in ItemCategory field (or the field you use for the catalog connection).
Create Content Search Web Part replacements
This is the most annoying part, we have to create a Web Part that we will use in category pages.
This web part does essentially three things:

1. Recover the current navigation term
2. Make a query searching che term related to the navigation term, in the MultivalueItemCategory we defined above
3. Substitute Path values (catalog item friendly url) in order to make links relative to the current navigation term
4. Show results

The core methods are About the single product page you can create a web part or a page layout, which has to contain methods similar to the previous web part.
This web part, must get the product using the navigation term and the friendly url segment and show the result.

Editing publishing site 
In pages document library, you have to create a category page (which contains the category CSWP replacement), and a product page (which contains the product CSWP replacement) and then edit target page settings for terms like this

 

And that's all folks.