Active Record Notes

gistfile1.rb

1. Retrieving Objects from the Database
Finder methods. All return an instance of ActiveRecord::Relation:
  bind
  create_with
  eager_load
  extending
  from
  group
  having
  includes
  joins
  limit
  lock
  none
  offset
  order
  preload
  readonly
  references
  reorder
  reverse_order
  select
  distinct
  uniq
  where
1.1 Retrieving a Single Object
  - using a primary Key - client = Client.find(10)
  # finds a Client object with id: 10
  - find_by - Client.find_by first_name: 'Lifo'
  # Client with a first name of 'Lifo'
1.2 Retrieving Multiple Objects
  - Using Multiple primary keys - Client.find([1,10])
  # Finds Clients with ids 1 and 10
  - take - Client.take(2) (first(2))
  # Takes first 2 Client records
1.3 Retrieving Multiple Objects in Batches
  # This is very inefficient when the users table has thousands of rows.
  User.all.each do |user|
    NewsLetter.weekly_deliver(user)
  end
  - Instructs Active Record to fetch the entire table in a single pass, build a model object
  per row, then keep the entire array of model objects in memory.
  User.find_each do |user|
    NewsLetter.weekly_deliver(user)
  end
  - Retrieves a batch of records and then yields each record to the block individually as a model
  - Change default amount of records using batch_size: User.find_each(batch_size: 5000)
  # Give add_invoices an array of 1000 invoices at a time
  Invoice.find_in_batches(include: :invoice_lines) do |invoices|
    export.add_invoices(invoices)
  end
  - find_in_batches works like find_each, but returns the whole batch of records at a time,
  instead of individually.
  - :include allows you to name associations that should be loaded alongside the models.
  
  2. Conditions
    2.1 Pure String Conditions
    - Client.where("orders_count = '2'") - find all clients where orders_count field's value is 2
    2.2 Array Conditions
    - Client.where("orders_count = ?", params[:orders]) - will go through the first element 
    in the conditions value and any additional elements
    - Client.where("orders_count = ? AND locked = ?", params[:orders], false) - specify multiple conditions
    This code is highly preferable:
      Client.where("orders_count = ?", params[:orders])
    to this code:
      Client.where("orders_count = #{params[:orders]}")