How does the params method work? - ruby-on-rails

Question

I have been trying to figure out how the params method works and I'm a bit stuck on the process.
For example, if a user clicks on a certain blog post in the index page, I guess that the link_to method calls the Post controller and the show action along with its block #post = Post.find(params[:id]) and then goes to the database to find the post and the view displays it.
So my missing link seems to be when is the post id passed into the params method?

Answer 1

Because the others already explained about params, I'm just going to answer directly a question of yours:
when is the post id passed into the params method
I think it's best explained with an example; see below:
say that you clicked a link:
/posts/1/?param1=somevalue1&param2=somevalue2
The Rails server receives this request that a client wants to view this GET /posts/1/?param1=somevalue1&param2=somevalue2 address.
To determine how the Rails server will respond, the server will first go to your routes.rb and find the matching controller-action that will handle this request:
# let's say your routes.rb contain this line
# resources :posts
# resources :posts above actually contains MANY routes. One of them is below
# For sake of example, I commented above code, and I only want you to focus on this route:
get '/posts/:id', to: 'posts#show'
From above notice that there is this :id, Rails will automatically set params[:id] to the value of this :id. This is the answer to your question where params[:id] comes from.
It doesn't have to be :id; you can name it whatever you want. You can even have multiple URL params like so (just an example):
get /users/:user_id/posts/:id which will automatically set the value on params[:user_id] and params[:id] respectively.
In addition to this URL params like :id, Rails also injects values to params[:controller] and params[:action] automatically from the routes. Say from the example above, get '/posts/:id', to: 'posts#show', this will set params[:controller] to 'posts', and params[:action] to 'show'.
params values also comes from other sources like the "Query string" as described by Mayur, and also comes from the body of the request, like when you submit a form (the form values are set within the body part of the request) and like when you have JSON requests, which all of these are automatically parsed by Rails for your convenience, so you could just simply access params and get the values as you need them.

Answer 2

Params are hashes in ruby with Indifferent access which means,
hsh = {"a" => 1, "b" => 2}
Consider this hsh as params returned from a POST request from browser, it's a key value pair with keys as string. Since it's a params so the values can be accessed as
hsh["a"]
=> 1
hsh [:a]
=> 1
params are formed on the client where the interface load, consider a form which has a submit button. When you press submit, the data filled in form or any hidden textboxes are formed into a hash and passed across the request. This when received on server end will be called as params or request params.
For Get requests: data send across the url will be read as params on backend.
GET: http://www.abx.com?user=admin
params on backend: {"user" => "admin"}
This will be displayed in rails server logs
For Put request: data send across the body will be called params.
PUT: http://www.abx.com
data: {"user" => "admin"} Client side
params on backend: {"user" => "admin"}
This will be displayed in rails server logs

Answer 3

How does the params method work?
The params come from the user's browser when they request the page. For an HTTP GET request, which is the most common, the params are encoded in the URL. For example, if a user's browser requested
http://www.example.com/?post=1&comment=demo
then params[:post] would be "1" and params[:comment] would be "demo".
In HTTP/HTML, the params are really just a series of key-value pairs where the key and the value are strings, but Ruby on Rails has a special syntax for making the params be a hash with hashes or array or strings inside.
It might look like this:
{"post"=>"1", "comment"=>"demo"}
Link to Rails Guides on params: guides