undefined method `first' for nil:NilClass on .each statement - ruby-on-rails

Question

I need to join two very large tables from a single database in Rails. To reduce the amount of database calls, I want to use the .includes method. However, when I use this on my query I get the following error:
undefined method `first' for nil:NilClass on <% table_data.each do |d| %>
I can't for the life of my understand where this is coming from and how to fix it.
My code:
class Planning < ApplicationRecord
has_many :timeline, :foreign_key => "pcnrtoev"
end
class Timeline < ApplicationRecord
belongs_to :Planning
end
class OverviewController < ApplicationController
def index
#table_data = Planning.select(:pcnrtoev).includes(:timeline)
end
end
index.html.erb
<%= render partial: 'datatable', locals: {table_data: #table_data} %>
_datatable.html.erb
***
<% table_data.each do |d| %>
-do things-
<% end %>
****
Edit
If I change my initial query to
#table_data = Planning.select(:pcnrtoev).includes(:timeline).limit(50)
it seems to work. I still can't get to the data however, as it throws up this:
undefined method `telco_code_in' for #<Timeline::ActiveRecord_Associations_CollectionProxy:0x00007f4880c13638>
at
<% table_data.each do |d| %>
<%=if d.timeline.any?
>> telco_code_in = d.timeline.telco_code_in
end %>
<%end%>
Edit
It seems there is something wrong with the information I get from the database, rather than from the query itself. If I limit the output to 5000, everything is fine. If I limit the output to 10.000, I get the "No first method on .each"-error. Could this be because the Planning table can have one or more rows in the Timeline table?
Planning schema
create_table "planning", id: :integer, options: "ENGINE=InnoDB DEFAULT CHARSET=utf8", force: :cascade do |t|
t.string "pcnrtoev", null: false
t.string "postcode", null: false
t.string "huisnr", null: false
t.string "toev"
t.index ["huisnr"], name: "huisnr"
t.index ["id"], name: "id"
t.index ["pcnrtoev"], name: "pcnrtoev"
t.index ["pcnrtoev"], name: "pcnrtoev_unique", unique: true
t.index ["postcode"], name: "postcode"
t.index ["toev"], name: "toev"
end
The Timeline table is a view instead of a table, so I don't have an actual schema for that. I scraped the code of everything that could cause the error, so this is my complete code for now.
Also, this is a pre-existing database, maybe that's useful information?

Answer 1

It is hard to give you right code without schema but I will do my best to explaining your mistake. So when you write
Planning.select(:pcnrtoev)
you are basically saying to database. Hey give me all pcnrtoev columns from planing table. If you paste this code in console you will se something like this
<Planning id: nil, pcnrtoev: INT>,
<Planning id: nil, pcnrtoev: INT2>...
So your database returns you only pcnrtoev columns and all other columns are nill.(Including columns from timeline table).
I hope you see what went wrong.
If you still have problem with writing the query, give me schema of Planing and Timeline table.