An example of how I test CSV generation

test_csv.rb

class Person < ActiveRecord::Base
  #... some other methods ...
  
  def self.export_as_csv
    #... generates a CSV string ...
  end
end

#spec
describe Person do
  # 1. Doing a array.join(',') to make the spec more readble, 
  # instead of creating a stirng directly like this: "1,Bob,50"
  # 2. This spec becomes more and more unreadable as more columns are added.
  describe '.export_as_csv' do
    before do
      #... some other stuff...  
    end
    
    it 'should should return a csv' do
      csv = ['Id', 'Name', 'Age'].join(',') + "\n" + 
        [1, 'Bob', 50].join(',') + "\n" + 
        [2, 'Alice', 51].join(',')
      Person.export_as_csv.should == csv
    end
  end
end

My initial reaction is to say, this should be pulled into a new class or module. It doesn't really feel the responsibility of a database backed model. With that being said, that doesn't help or address the testing issue. Since this is Rails and you already have ActiveSupport loaded, I may suggest something like:

describe Person do

  it "exports the default fields to csv" do
    # I'd really prefer a stub of what gets returned from the DB, but without
    # more details I can't add that.
    expected_csv = <<-CSV.strip_heredoc
      Id,Name,Age
      1,Bob,50
      2,Alice,51
    CSV
    expect(Person.export_as_csv).to eq expected_csv
  end

end

See the ActiveSupport monkey patch: String#strip_heredoc