How does the HTTP params Hash work?

I am working on a Rails engine. When I go from a Main Application route to an Engine route, I think I lose a HTTP param, which I need for a db query.

My questions are,

1) When and how is the HTTP params Hash modified?

2) Is it possible that switching between a application route and engine route effects the params Hash?

Weston Platter wrote in post #1073624:

I am working on a Rails engine. When I go from a Main Application route
to
an Engine route, I think I lose a HTTP param, which I need for a db
query.

*My questions are,*
*
*
*1) When and how is the HTTP params Hash modified?*

AFAIK that all happens here in the Rails/Rack middleware stack.

http://guides.rubyonrails.org/rails_on_rack.html#action-controller-middleware-stack

The middleware that parses the user's data into the params hash is:
ActionDispatch::ParamsParser

At least as I understand it.

*2) Is it possible that switching between a application route and engine
route effects the params Hash?*

Sorry, I've not yet created a Rails engine so I don't know the answer to
this part.