Merge pull request Shopify#1221 from Shopify/zoey/custom-apps

Add page to describe how to create sessions for Custom Apps

docs/usage/custom_apps.md

@@ -0,0 +1,75 @@
+# Custom Apps
+
+If you have followed instructions on creating [custom apps](https://help.shopify.com/current/manual/apps/app-types/custom-apps), you should be able to access your API token without having to go through the OAuth flow.
+
+You can follow instructions for [initializing the session object](#initializing-the-session-object) to construct the session object to be used in authenticated API calls to your store.  There are 2 methods to use the session object to make API calls:
+1. [Passing `session` object into each client request](#passing-session-object-into-each-client-request)
+2. [Setting `active_session` in `ShopifyAPI::Context`](#setting-active-session-in-shopifyapicontext)
+
+## Initializing the Session object
+Following is a basic example to construct a simple Session object. You can see full list of parameters for this object in the [class definition](https://github.com/Shopify/shopify-api-ruby/blob/main/lib/shopify_api/auth/session.rb)
+```ruby
+session = ShopifyAPI::Auth::Session.new(
+  shop: "#{your_shop_name}.myshopify.com"
+  access_token: "the_token_for_your_custom_app_found_in_admin"
+)
+
+```
+## Using `Session` to make API calls
+
+### Passing `session` object into each client request
+Example:
+```ruby
+def make_api_request(shop)
+  # 1. create session object
+    session = ShopifyAPI::Auth::Session.new(
+      shop: "#{your_shop_name}.myshopify.com"
+      access_token: "the_token_for_your_custom_app_found_in_admin"
+    )
+
+  # 2a. Create API client with the session information
+  # session must be type `ShopifyAPI::Auth::Session`
+  graphql_client = ShopifyAPI::Clients::Graphql::Admin.new(session: session)
+  response = graphql_client.query(query: MY_API_QUERY)
+
+  # 2b. REST example
+  product_count = ShopifyAPI::Product.count(session: session)
+
+  ...
+end
+```
+
+### Setting `active_session` in `ShopifyAPI::Context`
+Alternatively, if you don't want to keep having to create/retrieve a Session object for a shop, you may set [`ShopifyAPI::Context.active_session`](https://github.com/Shopify/shopify-api-ruby/blob/main/lib/shopify_api/context.rb#L144).
+All of the API client classes will [use the `active_session`](https://github.com/Shopify/shopify-api-ruby/blob/c3bb9d8f8b6053756149a4cf9299e059ec337544/lib/shopify_api/clients/http_client.rb#L13) if the `session` passed in is `nil`.
+
+Example:
+```ruby
+#### Configuration
+def configure_app
+  # This method is called before making authenticated API calls
+  session = ShopifyAPI::Auth::Session.new(
+      shop: "#{your_shop_name}.myshopify.com"
+      access_token: "the_token_for_your_custom_app_found_in_admin"
+    )
+
+  # Activate session to be used in all API calls
+  # session must be type `ShopifyAPI::Auth::Session`
+  ShopifyAPI::Context.activate_session(session)
+end
+
+#### Using clients to make authenticated API calls
+def make_api_request
+  # 1. Create API client without session information
+  # The graphql_client will use `ShopifyAPI::Context.active_session` when making API calls
+  graphql_client = ShopifyAPI::Clients::Graphql::Admin.new
+
+  # 2. Use API client to make queries
+  ...
+end
+
+```
+
+⚠️ See following docs on how to use the API clients:
+- [Make a GraphQL API call](https://github.com/Shopify/shopify-api-ruby/blob/main/docs/usage/graphql.md)
+- [Make a REST API call](https://github.com/Shopify/shopify-api-ruby/blob/main/docs/usage/rest.md)

docs/usage/graphql.md

@@ -6,6 +6,7 @@ Every API request requires a valid
 [ShopifyAPI::Auth::Session](https://github.com/Shopify/shopify-api-ruby/blob/main/lib/shopify_api/auth/session.rb).
 
 To instantiate a session, we recommend you either use the `shopify_app` if working in Rails, or refer to our OAuth docs on constructing a session:
+ - ["Custom Apps"](https://github.com/Shopify/shopify-api-ruby/blob/main/docs/usage/custom_apps.md) - documentation on how to create Session from a custom app API token.
  - ["Performing OAuth"](https://github.com/Shopify/shopify-api-ruby/blob/main/docs/usage/oauth.md) - documentation on how to create new sessions
  - [[ShopifyApp] - "Session"](https://github.com/Shopify/shopify_app/blob/main/docs/shopify_app/sessions.md) - documentation on session handling if you're using the [`ShopifyApp`](https://github.com/Shopify/shopify_app) gem.
 

docs/usage/rest.md

@@ -6,6 +6,7 @@ Every API request requires a valid
 [ShopifyAPI::Auth::Session](https://github.com/Shopify/shopify-api-ruby/blob/main/lib/shopify_api/auth/session.rb).
 
 To instantiate a session, we recommend you either use the `shopify_app` if working in Rails, or refer to our OAuth docs on constructing a session:
+ - ["Custom Apps"](https://github.com/Shopify/shopify-api-ruby/blob/main/docs/usage/custom_apps.md) - documentation on how to create Session from a custom app API token.
  - ["Performing OAuth"](https://github.com/Shopify/shopify-api-ruby/blob/main/docs/usage/oauth.md) - documentation on how to create new sessions
  - [[ShopifyApp] - "Session"](https://github.com/Shopify/shopify_app/blob/main/docs/shopify_app/sessions.md) - documentation on session handling if you're using the [`ShopifyApp`](https://github.com/Shopify/shopify_app) gem.
 
@@ -39,6 +40,8 @@ To instantiate a session, we recommend you either use the `shopify_app` if worki
 ## Using REST Resources
 We provide a templated class library to access REST resources similar to `ActiveResource`. Format of the methods closely resemble our [REST API schema](https://shopify.dev/docs/api/admin-rest).
 
+The version of REST resource that's loaded and used is set from [`ShopifyAPI::Context.setup`](https://github.com/Shopify/shopify-api-ruby/blob/main/README.md#setup-shopify-context)
+
 ### Instantiation
 Create an instance of the REST resource you'd like to use and optionally provide the following parameters.
 #### Constructor parameters