We got mass insertion working!

sales_engine_active_record_mass_insertion.rb

# s_arb
require 'active_record'
ActiveRecord::Base.establish_connection adapter: 'sqlite3', database: ':memory:'

ActiveRecord::Schema.define do
  self.verbose = false
  
  create_table :customers do |t|
    t.string :first_name
    t.string :last_name
    t.timestamps
  end
  
  create_table :invoices do |t|
    t.integer :customer_id
    t.integer :merchant_id
    t.string  :status
    t.timestamps
  end
end

class Customer < ActiveRecord::Base
  has_many :invoices
end

class Invoice < ActiveRecord::Base
  belongs_to :customer
end


require 'csv'
def to_sql_ary(values)
  elements = values.map { |value| ActiveRecord::Base.connection.quote value }
  "(" + elements.join(', ') + ")"
end

def insert_rows(tablename:, colnames:, rows:)
  rows_sql = rows.map { |values| "  #{to_sql_ary values}" }.join(",\n")
  ActiveRecord::Base.connection.execute <<-SQL
    INSERT INTO #{tablename} #{to_sql_ary colnames}
    VALUES #{rows_sql}
  SQL
end

def csvname_for(name)
 "/Users/josh/deleteme/sales_engines/sally-justin/data/#{name}.csv"
end

def load_csv(name)
  CSV.foreach(csvname_for(name), headers: true, header_converters: :symbol)
     .each_slice(200) { |rows|
       insert_rows tablename: name,
                   colnames:  rows.first.headers,
                   rows:      rows.map { |row| row.to_h.values }
     }
end

load_csv 'customers'
load_csv 'invoices'

Customer.find_by(id: 999) # => #<Customer id: 999, first_name: "Clementina", last_name: "Hudson", created_at: "2012-03-27 14:58:15", updated_at: "2012-03-27 14:58:15">
        .invoices         # => #<ActiveRecord::Associations::CollectionProxy [#<Invoice id: 4833, customer_id: 999, merchant_id: 59, status: "shipped", created_at: "2012-03-17 19:58:15", updated_at: "2012-03-17 19:58:15">, #<Invoice id: 4834, customer_id: 999, merchant_id: 75, status: "shipped", created_at: "2012-03-19 10:58:15", updated_at: "2012-03-19 10:58:15">, #<Invoice id: 4835, customer_id: 999, merchant_id: 58, ...
        .count            # => 7

Customer.count # => 1000
Customer.first
# => #<Customer:0x007fca4a0f2e38
#     id: 1,
#     first_name: "Joey",
#     last_name: "Ondricka",
#     created_at: 2012-03-27 14:54:09 UTC,
#     updated_at: 2012-03-27 14:54:09 UTC>