Fixing Rails’ “undefined method `map’ for nil:NilClass” errors for collection_selects

I had a weird problem. I defined a collection_select in my form, something like this:

<%= f.collection_select :campaign_id, @campaigns, :id, :title %>

When I went to the form, the page loaded, and the select populated correctly. However, when I submitted the form, I would get the “undefined method `map’ for nil:NilClass” error.

By mistake, I realized that the sometimes the form posted and sometimes it didn’t. Then I realized that there were Active Record validation errors with the submitted form, but the page wasn’t showing, so I couldn’t see that these errors occurred (even while looking at the logs).

I had to search how to fix this, and eventually found this Stack Overflow question.

The solution by Wes Foster, worked for me to ensure that the collection was getting determined when the new action was rendered (after the record didn’t create).

class MyController < ApplicationController
  before_action :set_select_collections, only: [:edit, :update, :new, :create]

    def set_select_collections
      @campaigns =

Incidentally, if you’re receiving this error and your form isn’t even loading, it probably means that you didn’t define the collection value in your controller (@campaigns in this example).

Changing Column to Add Precision and Scale

If you have to add precision and scale to a decimal field, here’s a sample migration:


class ChangeScale < ActiveRecord::Migration
def self.up
change_column :shirts, :price, :decimal, precision: 5, scale: 2

def self.down
change_column :shirts, :price, :decimal, :precision, :scale

Sorting Nil Results to End when Order Ascending (ASC)

I had to sort a bunch of records with order ascending, but the problem is that some of these records will occasionally be nil.   For my use case, I needed the nils to be listed at the end, with the non-nils sorted in ascending order.

So I had something like this:

@bars = @foo.bars.order(“number_of_votes ASC”)

where I wanted the articles to show in the order of votes they received, but I wanted the ones with a nil number of votes to come last.

I found the answer in this Stack Overflow question using coalesce.

(1) First, I needed to determine the number of records (i.e. count = @foo.bars.count)

(2) Then I adjusted my code to:

count = @foo.bars.count

@bars = @foo.bars.order(“coalesce(number_of_votes, #{count}) ASC”)

This treated all of the nil values as if it had a number_of_votes, placing them at the end of the array.

Rails Queries not Finding Nil Values

I had some queries that were not working and I couldn’t figure out how to make them work.  Basically, I was looking for fields with nil values, so I tried:

Model.where(‘variable = ?’, nil)….and didn’t get any results

So just for the heck of it, I tried:

Model.where(‘variable != ?’, nil)…but still no results.  How is that possible?  It either has to be a value or it isn’t right?

Well, apparently you can’t use normal comparison operands with nil/null.  However, instead of =, you can compare for nil using <=>

So I ended up doing:

Model.where(‘variable <=> ?’, nil)

and it worked, finding the nil values.

Setting Field to Null in MySQL Workbench

I had some bad data in my DB where fields had a space or some other value I could not see, so I needed to set the field to null (my program looks for null).  To do this, I changed the value of the field, then clicked “Apply” and on the SQL preview screen, I changed the value to NULL (ie city = NULL where…).

Lessons Importing CSV’s into PHPMYADMIN

I’ve been working on building dummy data into my local database.  Uploading a CSV that I filled with data using Excel, I populated my database.  Here are two lessons I learned.

Null Integers?
For one of the fields that I didn’t populate with data, imported a 0 because it was an integer field.  However, I needed the field null.  When I went into the record and took out the 0 and tried to save it, I got an error.  What I found was that when I clicked the checkbox for null for the record, it nullified the field.

Date Format
The date format was the European format in my database YYYY-MM-DD.  However, when I had the dates in the American format in my CSV MM-DD-YYYY. The records imported successfully in my database, but instead of erring out with the data, it set the date to 0000-00-00.  So I just had to delete the imported records, and then edit the date format in the CSV and re-import.