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Querying with Rails
By Parth Modi

Index
● ORM - What, Why
● About ActiveRecord
● Migrations
● Models
● Relationships
● Queries
● Scopes
● What I learned so far

Object Relational Mapping
“Connects the rich objects of an application to tables in a relational database
management system.”
● No need to depend on Database
● No need to write SQL statements directly (Most of the time)
● Database Connection - Implementation Abstracted
● Lets you focus on Application

Active Record
“Objects carry both persistent data and behavior which operates on that data. Active
Record takes the opinion that ensuring data access logic as part of the object will
educate users of that object on how to write to and read from the database.”
● Associations
● Validations
● Different Database Operations in Object Oriented fashion
● Inheritance Hierarchies

Migrations
● DDL commands
● Track changes in table schema, with time
● Never delete a migration script
● Know when to use rails db:migrate and when to rails db:schema:load

Models
● ActiveRecord backed
● Model lets you create objects that represents row of the table in database
● Attributes, Associations, Validations, Callbacks
● Conventions over Configuration
● http://guides.rubyonrails.org/active_record_basics.html

Associations
State
District District District
Pincode Pincode Pincode

Queries
● #find, #find_by
● #select
● #where, #rewhere
● #order
● #group, #having
● #join, #left_outer_join

Queries - tends to get complex over time
● Get pincodes:
○ SELECT “pincodes”.* FROM “pincodes”;
● Sort Pin Codes by code:
○ SELECT “pincodes”.* FROM “pincodes” ORDER BY "pincodes"."code" ASC;
● Apply search:
○ SELECT "pincodes".*
FROM "pincodes"
WHERE (code ILIKE '%SEARCH-TERM%')
ORDER BY "pincodes"."code" DESC
LIMIT 10
OFFSET 0;
● ActiveRecord sums all of this into:

Queries - and even before your app hits
production● Get Pincodes from A certain State:
SELECT "pincodes".*
FROM "pincodes"
INNER JOIN "districts" ON "districts"."id" = "pincodes"."district_id"
INNER JOIN "states" ON "states"."id" = "districts"."state_id"
WHERE (code ILIKE '%SEARCH-TERM%') AND (states.name = 'Gujarat')
ORDER BY "pincodes"."code" DESC
LIMIT 10
OFFSET 0;

Queries - Readable? Concise?
● ActiveRecord equivalent of previous query:
Pincode.joins(district: [:state])
.where(“states.name = ? ”, “Gujarat”)
.where('code ILIKE ?', '%SEARCH-TERM%')
.order(code: :desc)
.limit(10)
.offset(0)

● This goes on and on...
Queries - pagination, search, ordering, what not!

Scopes to the rescue
● Lambda Functions, with names ( ? )
● Always returns ActiveRecord::Relation object
● You can pass arguments as you would with normal methods
● Holds chunks of query logic
● Chainable
● http://guides.rubyonrails.org/active_record_querying.html#scopes
● https://www.justinweiss.com/articles/should-you-use-scopes-or-class-methods/

Pincode model After Applying scopes

Querying - with Scopes!
● And the query in becomes:
○ Pincode.by_state('Gujarat').order_by('code').search('x').limit(10).offset(10)

Tips
● Fetch only data you need
● Ruby Methods vs Query Methods
● Use IN query
● Wrap batch processing inside transaction
● Never use #all, use #find_each
● Use #joins, #includes and #preload frequently
● https://www.sitepoint.com/n-1-when-more-queries-is-a-good-thing/
● https://blog.codeship.com/writing-efficient-queries/

Gracias!
Follow my articles at: parthrmodi.com/blog and github at github.com/8parth

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