Add /api/v1_alpha/accounts/:id/in_collections to list collections you are in (#38657)

app/controllers/api/v1_alpha/in_collections_controller.rb

@@ -0,0 +1,63 @@
+# frozen_string_literal: true
+
+class Api::V1Alpha::InCollectionsController < Api::BaseController
+  include Authorization
+
+  DEFAULT_COLLECTIONS_LIMIT = 40
+
+  before_action :check_feature_enabled
+
+  before_action -> { authorize_if_got_token! :read, :'read:collections' }, only: [:index]
+
+  before_action :require_user!
+  before_action :set_account, only: [:index]
+  before_action :set_collections, only: [:index]
+
+  after_action :insert_pagination_headers, only: [:index]
+
+  after_action :verify_authorized
+
+  def index
+    cache_if_unauthenticated!
+    authorize @account, :index_featured_in_collections?
+
+    render json: @collections, each_serializer: REST::CollectionSerializer, adapter: :json
+  end
+
+  private
+
+  def set_account
+    @account = Account.find(params[:account_id])
+  end
+
+  def set_collections
+    @collections = @account.featured_in_collections
+      .with_tag
+      .offset(offset_param)
+      .limit(limit_param(DEFAULT_COLLECTIONS_LIMIT))
+  end
+
+  def check_feature_enabled
+    raise ActionController::RoutingError unless Mastodon::Feature.collections_enabled?
+  end
+
+  def next_path
+    return unless records_continue?
+
+    api_v1_alpha_account_in_collections_url(@account, pagination_params(offset: offset_param + limit_param(DEFAULT_COLLECTIONS_LIMIT)))
+  end
+
+  def prev_path
+    return if offset_param.zero?
+
+    api_v1_alpha_account_in_collections_url(@account, pagination_params(offset: offset_param - limit_param(DEFAULT_COLLECTIONS_LIMIT)))
+  end
+
+  def records_continue?
+    ((offset_param * limit_param(DEFAULT_COLLECTIONS_LIMIT)) + @collections.size) < @account.featured_in_collections.size
+  end
+
+  def offset_param
+    params[:offset].to_i
+  end
+end