Mongoid Aggregate result into an instance of a rails model - ruby-on-rails

Question

Introduction
Correcting a legacy code, there is an index of object LandingPage where most columns are supposed to be sortable, but aren't. This was mostly corrected, but few columns keep posing me trouble.
Theses columns are the one needing an aggregation, because based on a count of other documents. To simplify the explanation of the problem, I will speak only about one of them which is called Visit, as the rest of the code will just be duplication.
The code fetch sorted and paginate data, then modify each object using LandingPage methods before sending the json back. It was already like this and I can't modify it.
Because of that, I need to do an aggregation (to sort LandingPage by Visit counts), then get the object as LandingPage instance to let the legacy code work on them.
The problem is the incapacity to transform Mongoid::Document to a LandingPage instance
Here is the error I got:
Mongoid::Errors::UnknownAttribute:
Message:
unknown_attribute : message
Summary:
unknown_attribute : summary
Resolution:
unknown_attribute : resolution
Here is my code:
def controller_function
landing_pages = fetch_landing_page
landing_page_hash[:data] = landing_pages.map do |landing_page|
landing_page.do_something
# Do other things
end
render json: landing_page_hash
end
def fetch_landing_page
criteria = LandingPage.where(archived: false)
columns_name = params[:columns_name]
column_direction = params[:column_direction]
case order_column_name
when 'visit'
order_by_visits(criteria, column_direction)
else
criteria.order_by(columns_name => column_direction).paginate(
per_page: params[:length],
page: (params[:start].to_i / params[:length].to_i) + 1
)
end
def order_by_visit(criteria, order_direction)
def order_by_visits(landing_pages, column_direction)
LandingPage.collection.aggregate([
{ '$match': landing_pages.selector },
{ '$lookup': {
from: 'visits',
localField: '_id',
foreignField: 'landing_page_id',
as: 'visits'
}},
{ '$addFields': { 'visits_count': { '$size': '$visits' }}},
{ '$sort': { 'visits_count': column_direction == 'asc' ? 1 : -1 }},
{ '$unset': ['visits', 'visits_count'] },
{ '$skip': params[:start].to_i },
{ '$limit': params[:length].to_i }
]).map { |attrs| LandingPage.new(attrs) { |o| o.new_record = false } }
end
end
What I have tried
Copy and past the hash in console to LandingPage.new(attributes), and the instance was created and valid.
Change the attributes key from string to symbole, and it still didn't work.
Using is_a?(hash) on any element of the returned array returns true.
Put it to json and then back to a hash. Still got a Mongoid::Document.
How can I make the return of the Aggregate be a valid instance of LandingPage ?

Answer 1

Aggregation pipeline is implemented by the Ruby MongoDB driver, not by Mongoid, and as such does not return Mongoid model instances.
An example of how one might obtain Mongoid model instances is given in documentation.