Advanced Active Record Notes

advanced-active-record.md

#Advanced Active Record

#Bullet Points

• Behaviours of Great Developers Active Record
  • shoulda-matchers

group :test do
  gem 'shoulda-matchers', require: false
end

• Console logging - SQL to STDOUT

  • ActiveRecord::Base.logger = Logger.new(STDOUT)

• Relationships

  • One to Many
  • Has and Belongs To Many vs Has Many Through
  • Non-Convention Named associations
    • manager_id -> Employee

• Custom Model Methods

  • Simple Transformations
  • Complex Lookups - Recursive

• scopes

  • ActiveRecord::Relation
  • .to_sql / .explain
  • .exists?
  • .unscope / .except / .only / reorder / rewhere / none
  • .readonly
  • includes(:albums) eager load use as array

• find / where / find_each(batch_size: 5000) / find_in_batches(include: :albums)

• order, where(:attr =>[x,y,z]), not, limit, offset, select + group / having

• Band.joins(:albums).select('bands.id','count(albums.id)','bands.name').group('bands.id').to_sql

• Band.all.joins(:albums).select('bands.id','count(albums.id)','bands.name').group('bands.id').pluck('bands.id','bands.name','count(albums.id)')

• https://github.com/rails/arel

• arel: Band.arel_table.table_name

  • /work/projects/arel_notes/postings
  • psql
  • \connection postings_development

#Demo / Live-code Overview ##Demo Resources

• active-record-example
• gem "sinatra-activerecord"

#Related Challenges

• TODO Challenge Name
  • Common Sticking Point 1
  • Common Sticking Point 2

##Demo Steps / Script

• TODO
• band.rb
• agent.try(:full_name)
• scope
• rake console
• Band.first.tracks.class

#Related Docs

• http://guides.rubyonrails.org/active_record_querying.html
• http://guides.rubyonrails.org/association_basics.html
• Rails Case Active Relation
• ActiveRecord Associations
• Sinatra How do I use ActiveRecord migrations?
• Create Table API
• Polymorphic Associations

#Original Notes

use the associational interface

1. quickly walk through an example many-to-many association. Employees have employee_meetings, meetings have employee_meetings, employee_meetings belong to employees and meetings. models and migrations are already written.

2. use driver_code.rb for examples. some code already there, commented out. * basic access from collections (employee.meetings) * never reference a foreign key directly. (don't talk to employee_meetings!) walk through three ways to create. * chaining methods when necessary. where, clear (employee.meetings.where().clear) (clear removes associations)

3. use of complex, named associations when necessary. * employees have a manager_id column, which contains a foreign_key reference to the employee table. add manager_id column migration file, talk about adding instead of dropping databases. * add the manager association to the employee table. code is at the bottom of driver_code.rb. walk through some basic driver code. * add the underlings association (employees have many underlings). * talk about relying on the associational interface for queries. it can handle a lot! so what logic should be done in custom methods? two big categories:

   • really simple attribute transforms, often for display. so something like a short name or a all capitals version of something.
   • really complex logic, like recursively finding an employees boss' boss' boss.

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