Conditional scope example in rails

model.rb

Some Model Name
  ...
  
  # scopes
  scope :by_category, ->(category) { where(category: category) if category.present? }
  scope :by_project_id, ->(id) { where(project_id: id ) }

  ...
  
  # class methods
  def self.by_params(category, project_id = nil) 
    docs = ProjectDoc.by_category(category)
    docs.by_project_id(project_id) if project_id.present?
    docs 
  end
  
  ...
  
end

Inspiration:
https://gistpages.com/posts/conditionally-chain-scopes-in-rails