support ?user_id in list admins endpoint

fixes CAT-225

test plan:
* GET /api/v1/accounts/:account_id/admins?user_id[]=123
* hit the above endpoint for an account with admins.
  change the 123 user_id to the id of a user with admin rights.
* verify that the result is just the admin user for that user
* hit the same endpoint with some nonadmin user
* verify that the result is []
* verify that if you send in multiple user_ids, you receive
  admins for all those user_ids
  example:
  GET /api/v1/accounts/:account_id/admins?user_id[]=1&user_id[]=2
  should return admins for that account_id with user_id's 1 or 2

Change-Id: I533135d80dfbe359d96f750e71a59fe714a67e98
Reviewed-on: https://gerrit.instructure.com/35888
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Dave Donahue <ddonahue@instructure.com>
Product-Review: Marc LeGendre <marc@instructure.com>
QA-Review: Marc LeGendre <marc@instructure.com>

app/controllers/admins_controller.rb

@@ -54,7 +54,7 @@ class AdminsController < ApplicationController
   #
   # Flag an existing user as an admin within the account.
   #
-  # @argument user_id [String]
+  # @argument user_id [Integer]
   #   The id of the user to promote.
   #
   # @argument role [Optional, String]
@@ -97,10 +97,14 @@ class AdminsController < ApplicationController
   #
   # List the admins in the account
   #
+  # @argument user_id[] [Optional, [Integer]]
+  #   Scope the results to those with user IDs equal to any of the IDs specified here.
+  #
   # @returns [Admin]
   def index
     if authorized_action(@context, @current_user, :manage_account_memberships)
       scope = @context.account_users
+      scope = scope.where(user_id: params[:user_id]) if params[:user_id]
       route = polymorphic_url([:api_v1, @context, :admins])
       admins = Api.paginate(scope.order(:id), self, route)
       render :json => admins.collect{ |admin| admin_json(admin, @current_user, session) }

spec/apis/v1/admins_api_spec.rb

@@ -240,6 +240,48 @@ describe "Admins API", type: :request do
                              "login_id"=>@admin.pseudonym.unique_id}}]
       end
 
+      it "should scope the results to the user_id if given" do
+        2.times do |x|
+          u = user(:name => "User #{x}", :account => @account)
+          @account.account_users.create!(:user => u, :membership_type => "MT #{x}")
+        end
+        @user = @admin
+        json = api_call(:get, @path, @path_opts.merge(user_id: @admin.id))
+        json.should ==[{"id"=>@admin.account_users.first.id,
+                        "role"=>"AccountAdmin",
+                        "user"=>
+                            {"id"=>@admin.id,
+                             "name"=>@admin.name,
+                             "sortable_name"=>@admin.sortable_name,
+                             "short_name"=>@admin.short_name,
+                             "login_id"=>@admin.pseudonym.unique_id}}]
+      end
+
+      it "should scope the results to the array of user_ids if given" do
+        2.times do |x|
+          u = user(:name => "User #{x}", :account => @account)
+          @account.account_users.create!(:user => u, :membership_type => "MT #{x}")
+        end
+        another_admin = @user
+        @user = @admin
+        json = api_call(:get, @path, @path_opts.merge(user_id: [@admin.id, another_admin.id]))
+        json.should ==[{"id"=>@admin.account_users.first.id,
+                        "role"=>"AccountAdmin",
+                        "user"=>
+                            {"id"=>@admin.id,
+                             "name"=>@admin.name,
+                             "sortable_name"=>@admin.sortable_name,
+                             "short_name"=>@admin.short_name,
+                             "login_id"=>@admin.pseudonym.unique_id}},
+                       {"id"=>another_admin.account_users.first.id,
+                        "role"=>"MT 1",
+                        "user"=>
+                            {"id"=>another_admin.id,
+                             "name"=>another_admin.name,
+                             "sortable_name"=>another_admin.sortable_name,
+                             "short_name"=>another_admin.short_name}}]
+      end
+
       it "should paginate" do
         4.times do |x|
           u = user(:name => "User #{x}", :account => @account)