MongoDB Aggregation Mechanism - Renaming a Dynamic Field

I believe that the mongodb aggregation structure is extremely efficient - it seems like a good opportunity to smooth out an object. My schema uses an array of helper objects in an array called materials. The number of materials is variable, but a specific field (category) will be unique to objects in the array. I would like to use the aggregation structure to smooth the structure and dynamically rename the fields based on the value of the category field. I could not find an easy way to accomplish this using $ project along with $ cond. Is there any way?

The reason for the array of material objects is a simple search:

eg. {'materials.name': 'XYZ'} discards any document in which XYZ is found.

eg. before and after the document

{
"_id" : ObjectId("123456"),
"materials" : [
    {
        "name" : "XYZ",
        "type" : "Red",
        ...
        "category" : "A"
    },
    {
        "name" : "ZYX",
        "type" : "Blue",
        ...
        "category" : "B"
    }]
}

{
"material_A_name" : "XYZ",
"material_A_type" : "Red",
...
"material_B_name" : "ZYX",
"material_B_type" : "Blue",
...
}