What Am I doing wrong in the has many through association In Rails 6?

So, I have 3 models(account, user_category, category) every model is inside different folders.
I've defined a has_many through association to associate Account to Category through middle table User Category so that a user can have many categories but it is not working as intended.

Account Model

module AccountBlock
  class Account < AccountBlock::ApplicationRecord
    ActiveSupport.run_load_hooks(:account, self)
    self.table_name = :accounts

    include Wisper::Publisher

    has_many :user_categories, class_name: "BxBlockCategories::UserCategory", dependent: :destroy
    has_many :categories, class_name: "BxBlockCategories::Category", through: "user_categories", dependent: :delete_all

end

User Category Model

module BxBlockCategories
  class UserCategory < ApplicationRecord
    self.table_name = :user_categories

    belongs_to :account, class_name: "AccountBlock::Account", foreign_key: :account_id
    belongs_to :category, class_name: "BxBlockCategories::Category", foreign_key: :category_id
  end
end

Category Model

module BxBlockCategories
  class Category < BxBlockCategories::ApplicationRecord
    self.table_name = :categories


    has_and_belongs_to_many :sub_categories,
                            join_table: :categories_sub_categories, dependent: :destroy

    has_many :contents, class_name: "BxBlockContentmanagement::Content", dependent: :destroy
    has_many :ctas, class_name: "BxBlockCategories::Cta", dependent: :nullify

    has_many :user_categories, class_name: "BxBlockCategories::UserCategory", dependent: :destroy
    has_many :accounts, class_name: "AccountBlock::Account", through: :user_categories
             
  end

It works if I do category.account but returns an empty Association for account.categories.

Why is that?

I would love someone can help me.

I've included I've tried so far in the code above.

The problem here isn't the assocations. It's that you're making a classic beginner misstake and calling an instance method on an whole collection of records.

Imagine you have an entire group of people in front of you and you ask "What is your name?" - how would that work? Are they all going to shout their names at the same time or just stare at your like you're an idiot?

If you want to get the categories for a given account you need to loop through the results:

AccountBlock::Account.first.categories.each do |category|
  puts category.name
end

The assocations are also very overcomplicated and you can get rid of a lot of over-configuration by just using the module nesting correctly:

module AccountBlock
  # ApplicationRecord is resolved as AccountBlock::ApplicationRecord 
  class Account < ApplicationRecord 
    ActiveSupport.run_load_hooks(:account, self)
    self.table_name = :accounts
    has_many :user_categories, 
      # good - we are referring to a constant in another module.
      class_name: "BxBlockCategories::UserCategory", 
      dependent: :destroy
    has_many :categories, 
      class_name: "BxBlockCategories::Category", 
      through: "user_categories", 
      dependent: :delete_all
 end
end

module BxBlockCategories
  class UserCategory < ApplicationRecord
    self.table_name = :user_categories
    # you don't need specify `foreign_key` when it can be derived from the name of the association. 
    belongs_to :account, class_name: 'AccountBlock::Account'
    belongs_to :category
  end
end

module BxBlockCategories
  class Category < ApplicationRecord
    self.table_name = :categories
    has_many :user_categories, dependent: :destroy
    has_many :accounts, 
      through: :user_categories,
      class_name: 'AccountBlock::Account'
  end
end

So, I figured out what I was doing wrong.

In the Account Model file when I used through: "user_categories" I used a string literal which was causing the issue. Instead I had to use a symbol like this through: :user_categories.