metaprogramming.rb

# Let's simulate a DripService object that will give us some output: 
class DripService
  include ActiveModel::Model
  attr_accessor :entry
  def call
    print "You passed in an entry: #{entry.email}"
  end
end

# Let's also simulate an Entry object: 
class Entry
  include ActiveModel::Model
  attr_accessor :email
end

entry = Entry.new(email: '[email protected]')
#<Entry:0x00007f9ca0818a00 @email="[email protected]">

# This is the end result that we're trying to achieve: 
DripService.new(entry: entry).call
# You passed in an entry: [email protected]=> nil

# Now let's play with constantize: 

# This will return a plain string. 
# If you try to do DripService things to the string you'll get an error, because the string does not respond to 'new'
"DripService"
# => "DripService"
"DripService".new(entry: entry)
# NoMethodError (undefined method `new' for "DripService":String)

# Constantize tries to find a declared constant with the name specified in the string. It raises a NameError when the name is not in CamelCase or is not initialized. 
DripService".constantize
# => DripService

# Initizlize a new DripService object: 
"DripService".constantize.new(entry: entry)
# => #<DripService:0x007fea9fcfd2d0 @entry=#<Entry id: nil, email: nil, form_id: nil, created_at: nil, updated_at: nil>>

"DripService".constantize.new(entry: entry).call
# You passed in an entry: [email protected]=> nil

# Dynamically instantiating a class based on the provider name: 

provider_name = "drip"
# => "drip"

# We need to capitalize the first letter: 
"#{provider_name.titleize}"
# => "Drip"

# Construct a string that ends with "Service", since that's the name of our class: 
"#{provider_name.titleize}Service"
# => "DripService"

# Now we can constantize that string to return an actual DripService object: 
"#{provider_name.titleize}Service".constantize
# => DripService

"#{provider_name.titleize}Service".constantize.new(entry: entry)
# => #<DripService:0x007fea9fc659a8 @entry=#<Entry id: nil, email: nil, form_id: nil, created_at: nil, updated_at: nil>>

"#{provider_name.titleize}Service".constantize.new(entry: entry).call
# You passed in an entry: [email protected]=> nil