undefined method `val' for #<Arel::Nodes::BindParam:0x007fa13ef32028> - ruby-on-rails

Question

I am new to rails, and cannot figure out that why do I get the error specified above when I try to create a new application after login.
Also, while I was looking at solutions online, I found there was something wrong with the foreign key. therefore, I removed the foreign key and added it again using migration. However, the schema.rb file that is shown below does not show an index, but does show Student_id column.
Questions :
Should creation of has_one and belongs_to association require one to mention it in routes.rb as well . ( I read online it does require, but why ? )
How to decide where to place belongs_to and has_one.
How to find out where the above error is generated from ?
Model Student ( created using devise )
class Student < ActiveRecord::Base
devise :database_authenticatable, :registerable,
:recoverable, :rememberable, :trackable, :validatable
has_one :student_application
end
StudentApplication ( Generate using scaffold)
class StudentApplication < ActiveRecord::Base
belongs_to :Student
end
routes.rb
Rails.application.routes.draw do
resources :student_applications
devise_for :students
root to: "student_applications#index"
end
In the controller student_application_controller.rb
def new
##student_application = StudentApplication.new
#student_application = current_student.build_student_application()
end
def create
##student_application = StudentApplication.new(student_application_params)
##new_student = Student.find(params[:id])
#student_application = current_student.create_student_application(student_application_params)
respond_to do |format|
if #student_application.save
format.html { redirect_to #student_application, notice: 'Student application was successfully created.' }
format.json { render :show, status: :created, location: #student_application }
else
format.html { render :new }
format.json { render json: #student_application.errors, status: :unprocessable_entity }
end
end
end
private
def student_application_params
params.require(:student_application).permit(:Student_id, :name, :phone, :email_id, :gpa)
end
schema.rb
create_table "student_applications", force: :cascade do |t|
t.string "name"
t.string "phone"
t.string "email_id"
t.decimal "gpa"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
t.integer "Student_id"
end
create_table "students", force: :cascade do |t|
t.string "email", default: "", null: false
t.string "encrypted_password", default: "", null: false
t.string "reset_password_token"
t.datetime "reset_password_sent_at"
t.datetime "remember_created_at"
t.integer "sign_in_count", default: 0, null: false
t.datetime "current_sign_in_at"
t.datetime "last_sign_in_at"
t.string "current_sign_in_ip"
t.string "last_sign_in_ip"
t.datetime "created_at"
t.datetime "updated_at"
end
add_index "students", ["email"], name: "index_students_on_email", unique: true
add_index "students", ["reset_password_token"], name: "index_students_on_reset_password_token", unique: true
end