How to list nodes that share only the first selected value in an entity reference field? I have entity reference field to a taxonomy vocabulary on a node, which is a multi value field. 

In Views, how do I list related nodes that share only the first selected category? 

Example: Suppose I have selected "category3" and "category4" for a node. I want to show other nodes which have selected "category3".
You should be able to set up something like what you are after with the following steps:


Create a view of content (nodes)
Add a contextual filter for NID with a that gets it's default value from "Content ID from url" (presuming the related products are to appear on the original products own page).
Add a relationship on field_category, call it something like "category", require this relationship
Add a filter of field_category:delta, value = 0
Add a relationship of "Content using field_category", require this relationship, use relationship: category, call this relationship something like "related"
Add another contextual filter that's also for NID with a that gets it's default value from "Content ID from url", use relationship: related, tick "Exclude" in the "More" fieldset at the bottom.
Now when you add your fields use relationship: related.