Rails Validation Error on existing data - ruby-on-rails

Question

I am building a simple expenses management app on rails 5.1.4. I am using the following five models.
Payees
class Payee < ApplicationRecord
has_many :expenses
validates :title, uniqueness: true, presence: true
end
Accounts
class Account < ApplicationRecord
before_save :update_balance
validates :balance, numericality: { greater_than_or_equal_to: 0 }
has_many :expenses
end
Budgets
class Budget < ApplicationRecord
belongs_to :categories
has_many :expenses, through: :categories
end
Categories
class Category < ApplicationRecord
validates :title, uniqueness: true, presence: true
has_many :expenses
has_one :budget
end
Expenses
class Expense < ApplicationRecord
belongs_to :categories
belongs_to :budgets
belongs_to :payees
belongs_to :accounts
validates :title, :value, presence: true
before_save :default_account
end
When I try to create a new expense I am facing a validation error
Validation failed: Categories must exist, Budgets must exist, Payees must exist, Accounts must exist
The issue is that all the above records exist. To explain my self let's say I am passing the params account_id: 1, payee_id: 1, category_id: 1. If I do:
Account.find(1) #=> Finds the record
Category.find(1) #=> also ok
Payee.find(1) #=> also ok
I am aware of the solution referred in this question (adding optional: true) but I don't get why I should do that while all of the above exist
Edit
The code that is raising the error is:
def create
#expense = Expense.create!(title: params[:expense]['title'],
value: params[:expense]['value'],
date: params[:expense]['date'],
comment: params[:expense]['comment'],
payee_id: params[:expense]['payee_id'],
category_id: params[:expense]['category_id'],
account_id: params[:expense]['account_id'])
end
The parameters that are passed through the form are
{"utf8"=>"✓",
"authenticity_token"=>"DWd1HEcBC3DhUahfOQcdaY0/oE+VHapxxE+HPUb0I6iSiqMxkz6l+vlK+1zhb66HnZ/vZRUVG4ojTdWUCjHtGg==",
"expense"=>{"title"=>"test", "value"=>"-20", "category_id"=>"1", "payee_id"=>"2", "date"=>"2018-01-21", "account_id"=>"1", "comment"=>""},
"commit"=>"Submit"}

Answer 1

I would first start by commenting out all your model validations, then creating an expense. Add back one model validation at a time, each time test creating an expense to see what validation is causing the error.
also you may want to change how you're creating the expense to something like below.
change your controllers create action to
def create
#expense = Expense.new(expense_params)
if #expense.save
flash[:success] = "expense created"
redirect_to expense_url(#expense.id)
else
render 'new'
end
end
next under your private method at the bottom of your controller you want to do something like this
private
# Never trust parameters from the scary internet, only allow the white list through.
def expense_params
params.require(:expense).permit(:title, :value, :date, etc...)
end

Answer 2

I finally found out where the problem is! It was the naming of the classes/models that raised the error. I had named my models on singular (Account, Category, etc) while all references are searching for plurals ( Accounts, Categories, etc). I had to re-do all migrations from the very beginning in order to make it work the proper way!
Thanks to everyone for spending the time though!