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Reflecting on
Active Record Associations
@composerinteralia
Daniel Colson
@dodecadaniel
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class Repository < ApplicationRecord
has_many :pull_requests
end
class PullRequest < ApplicationRecord
belongs_to :repository
end
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• repository
• repository=(repository)
• build_repository
• create_repository
• create_repository!
• reload_repository
• repository_changed?
• repository_previously_changed?
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• repository
• repository=(repository)
• build_repository
• create_repository
• create_repository!
• reload_repository
• repository_changed?
• repository_previously_changed?
• Presence Validations
• Caching
• Autosaving
• Preloading
• Destroy Callbacks
• Scopes
• Extensions
• Etc.
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Rails Magic !
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!!!!!!!
"#$%&'(
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Let’s Study Magic!
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@composerinteralia
Daniel Colson
@dodecadaniel
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No content
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Study Guide
• Metaprogramming
• Reflections
• Caching
• Relations
• Inverses
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Metaprogramming )
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Metaprogramming
class PullRequest < ApplicationRecord
belongs_to :repository
end
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Metaprogramming
class PullRequest < ApplicationRecord
self.belongs_to(:repository)
end
Calling a class method
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Metaprogramming
class PullRequest < ApplicationRecord
def self.belongs_to(name)
end
end
Defining a class method
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Metaprogramming
Reader Method
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Metaprogramming
> pull_request
=> <#PullRequest @repository_id=42>
Reader Method
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Metaprogramming
> pull_request
=> <#PullRequest @repository_id=42>
> pull_request.repository
=> <#Repository @id=42>
Reader Method
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Metaprogramming
> pull_request
=> <#PullRequest @repository_id=42>
> pull_request.repository
=> <#Repository @id=42>
Primary Key
Foreign Key
Reader Method
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Metaprogramming
def self.belongs_to(name)
def repository
Repository.where(id: repository_id).first
end
end
Reader Method
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Metaprogramming
Writer Method
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Metaprogramming
> pull_request
=> <#PullRequest @repository_id=42>
Writer Method
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Metaprogramming
> pull_request
=> <#PullRequest @repository_id=42>
> repository
=> <#Repository @id=77>
Writer Method
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Metaprogramming
> pull_request
=> <#PullRequest @repository_id=42>
> repository
=> <#Repository @id=77>
> pull_request.repository = repository
=> <#PullRequest @repository_id=77>
Writer Method
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Metaprogramming
def self.belongs_to(name)
def repository
Repository.where(id: repository_id).first
end
def repository=(repository)
self.repository_id = repository.id
end
end
Writer Method
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Metaprogramming
def self.belongs_to(name)
def repository
Repository.where(id: repository_id).first
end
def repository=(record)
self.repository_id = repository.id
end
end
Hardcoded
Hardcoded
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Metaprogramming
def self.belongs_to(name)
define_method(name) do
Repository.where(id: repository_id).first
end
define_method("#{name}=") do |record|
self.repository_id = repository.id
end
end
Generic
Generic
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Metaprogramming
def self.belongs_to(name)
define_method(name) do
Repository.where(:id => self[:repository_id]).first
end
define_method("#{name}=") do |record|
self[:repository_id] = record[:id]
end
end
Hardcoded
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def self.belongs_to(name)
define_method(name) do
klass.where(primary_key => self[foreign_key]).first
end
define_method("#{name}=") do |record|
self[foreign_key] = record[primary_key]
end
end
Metaprogramming
Generic
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Metaprogramming
class PullRequest < ApplicationRecord
belongs_to :repository
end
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Metaprogramming
class PullRequest < ApplicationRecord
belongs_to :repository
end
klass => Repository
primary_key => :id
foreign_key => :repository_id
?
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Reflections *
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Reflections
class Reflection
def initialize(name, active_record)
@active_record = active_record
@name = name
end
end
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Reflections
Reflection
@active_record = PullRequest
@name = :repository
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Reflections
• #klass
• #primary_key
• #foreign_key
Reflection
@active_record = PullRequest
@name = :repository
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Reflection
@active_record = PullRequest
@name = :repository
Reflections
• #klass => Repository
• #primary_key
• #foreign_key
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Reflection
@active_record = PullRequest
@name = :repository
Reflections
• #klass => Repository
• #primary_key
• #foreign_key
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Reflections
class Reflection
def klass
@name.to_s.camelize.constantize
end
end
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Reflections
• #klass => Repository
• #primary_key => :id
• #foreign_key
Reflection
@active_record = PullRequest
@name = :repository
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Reflections
class Reflection
def primary_key
klass.primary_key
end
end
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Reflection
@active_record = PullRequest
@name = :repository
Reflections
• #klass => Repository
• #primary_key => :id
• #foreign_key => :repository_id
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Reflection
@active_record = PullRequest
@name = :repository
Reflections
• #klass => Repository
• #primary_key => :id
• #foreign_key => :repository_id
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Reflections
class Reflection
def foreign_key
"#{@name}_id"
end
end
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Reflections
def self.belongs_to(name)
reflection = Reflection.new(name, self)
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Reflections
def self.belongs_to(name)
reflection = Reflection.new(name, self)
klass = reflection.klass
primary_key = reflection.primary_key
foreign_key = reflection.foreign_key
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def self.belongs_to(name)
define_method(name) do
klass.where(primary_key => self[foreign_key]).first
end
define_method("#{name}=") do |record|
self[foreign_key] = record[primary_key]
end
end
Reflections
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Caching +
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Caching
> repo = pull_request.repository
=> #
> same_repo = pull_request.repository
=> #
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Caching
> repo = pull_request.repository
=> #
> same_repo = pull_request.repository
=> #
Different objects
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Caching
> repo = pull_request.repository
=> #
> same_repo = pull_request.repository
=> #
> repo.name = "new_name"
> same_repo.name
=> "old_name"
Different objects
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Caching
> repo = pull_request.repository
=> #
> same_repo = pull_request.repository
=> #
> repo.name = "new_name"
> same_repo.name
=> "old_name"
Different objects
Inconsistent data ,
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Caching
class Association
def initialize(owner, reflection)
@owner = owner
@reflection = reflection
@loaded = false
@target = nil
end
end
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Caching
def self.belongs_to(name)
define_method(name) do
klass.where(primary_key => self[foreign_key]).first
end
define_method("#{name}=") do |record|
self[foreign_key] = record[primary_key]
end
end
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Caching
def self.belongs_to(name)
define_method(name) do
end
define_method("#{name}=") do |record|
end
end
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Caching
class Association
def reader
end
def writer(record)
end
end
Association Reader
Association Writer
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Caching
class Association
def reader
klass.where(primary_key => @owner[foreign_key]).first
end
def writer(record)
@owner[foreign_key] = record[primary_key]
end
end
Association Reader
Association Writer
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Caching
def self.belongs_to(name)
define_method(name) do
association(name).reader
end
define_method("#{name}=") do |record|
association(name).writer(record)
end
end
Use Association
Reader and Writer
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Caching
def self.belongs_to(name)
define_method(name) do
association(name).reader
end
define_method("#{name}=") do |record|
association(name).writer(record)
end
end
Use Association
Reader and Writer
Get Association by name
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Caching
Association
@owner = #
@reflection = #
@loaded = false
@target = nil
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Caching
def reader
if loaded?
else
end
end
Association
@owner = #
@reflection = #
@loaded = false
@target = nil
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Caching
def reader
if loaded?
else
end
end
Association
@owner = #
@reflection = #
@loaded = false
@target = nil
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Association
@owner = #
@reflection = #
@loaded = false
@target = nil
Caching
def reader
if loaded?
else
self.target = klass.where(...).first
end
end
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Association
@owner = #
@reflection = #
@loaded = false
@target = nil
Caching
def reader
if loaded?
else
self.target = klass.where(...).first
end
end
true
#
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Caching
def reader
if loaded?
else
self.target = klass.where(...).first
end
end
Association
@owner = #
@reflection = #
@loaded = true
@target = #
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Caching
def reader
if loaded?
target
else
self.target = klass.where(...).first
end
end
Association
@owner = #
@reflection = #
@loaded = true
@target = #
Association
@owner = #
@reflection = #
@loaded = true
@target = #
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Caching
def writer(record)
@owner[foreign_key] = record[primary_key]
self.target = record
end
Update target
Beware of a stale cache!
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Caching
> repo = pull_request.repository
=> #
> same_repo = pull_request.repository
=> #
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Caching
> repo = pull_request.repository
=> #
> same_repo = pull_request.repository
=> #
Same object
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Caching
> repo = pull_request.repository
=> #
> same_repo = pull_request.repository
=> #
> repo.name = "new_name"
> same_repo.name
=> "new_name"
Same object
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Caching
> repo = pull_request.repository
=> #
> same_repo = pull_request.repository
=> #
> repo.name = "new_name"
> same_repo.name
=> "new_name"
Same object
Consistent data -
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Relations .
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Relations
class Repository < ApplicationRecord
has_many :pull_requests
end
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def self.has_many(name)
define_method(name) do
association(name).reader
end
define_method("#{name}=") do |record|
association(name).writer(record)
end
end
Relations
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Relations
class BelongsToAssociation < Association
end
class HasManyAssociation < Association
end
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def reader
if loaded?
target
else
self.target =
klass.where(primary_key => @owner[foreign_key]).first
end
end
Relations
BelongsToAssociation#reader
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def reader
if loaded?
target
else
self.target =
klass.where(primary_key => @owner[foreign_key]).first
end
end
Relations
BelongsToAssociation#reader
Primary Key Foreign Key
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def reader
if loaded?
target
else
self.target =
klass.where(primary_key => @owner[foreign_key]).first
end
end
Relations
BelongsToAssociation#reader
Primary Key Foreign Key
Single Record
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def reader
if loaded?
target
else
self.target =
klass.where(foreign_key => @owner[primary_key]).to_a
end
end
Relations
HasManyAssociation#reader
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def reader
if loaded?
target
else
self.target =
klass.where(foreign_key => @owner[primary_key]).to_a
end
end
Relations
Foreign Key Primary Key
HasManyAssociation#reader
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def reader
if loaded?
target
else
self.target =
klass.where(foreign_key => @owner[primary_key]).to_a
end
end
Relations
Foreign Key Primary Key
Array of Records
HasManyAssociation#reader
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Relations
> repository.pull_requests
=> SELECT * FROM pull_requests WHERE repository_id = 42
=> [#]
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Relations
> repository.pull_requests
=> SELECT * FROM pull_requests WHERE repository_id = 42
=> [#]
> repository.pull_requests
=> [#]
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Relations
What is a Relation?
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Relations
> pull_requests = PullRequest.where(repository_id: 42)
=> <#ActiveRecord::Relation>
What is a Relation?
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Relations
> pull_requests = PullRequest.where(repository_id: 42)
=> <#ActiveRecord::Relation>
• Super-powered array of records
• Lazy loading
• #create
• And much more
What is a Relation?
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Relations
> pull_requests = repository.pull_requests
=> <#ActiveRecord::Relation>
Association Relations?
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Relations
> pull_requests = repository.pull_requests
=> <#ActiveRecord::Relation>
Association
@loaded = false
@target = nil
Association Relations?
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Relations
> pull_requests = repository.pull_requests
=> <#ActiveRecord::Relation>
> pull_requests.to_a
Association
@loaded = false
@target = nil
Association Relations?
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Relations
> pull_requests = repository.pull_requests
=> <#ActiveRecord::Relation>
> pull_requests.to_a
Association
@loaded = false
@target = nil
true
[<#PullRequest>]
=> [<#PullRequest>]
Association Relations?
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Relations
class CollectionProxy < ActiveRecord::Relation
def initialize(association)
@association = association
super
end
end
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def reader
CollectionProxy.new(self)
end
Relations
Updated Reader
HasManyAssociation#reader
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def load_target
if loaded?
target
else
self.target =
klass.where(foreign_key => @owner[primary_key]).to_a
end
end
Relations
HasManyAssociation#load_target
Moved From Reader
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HasManyAssociation
@loaded = false
@target = nil
repository.pull_requests
CollectionProxy
@association =
Relations
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HasManyAssociation
@loaded = false
@target = nil
repository.pull_requests
repository.pull_requests.to_a
CollectionProxy
@association =
Relations
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HasManyAssociation
@loaded = false
@target = nil
repository.pull_requests
repository.pull_requests.to_a
CollectionProxy
@association = true
[]
=> []
Relations
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repository.pull_requests
repository.pull_requests.to_a => []
repository.pull_requests.create
HasManyAssociation
@loaded = true
@target = []
Relations
CollectionProxy
@association =
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repository.pull_requests
repository.pull_requests.to_a => []
repository.pull_requests.create
HasManyAssociation
@loaded = true
@target = []
Relations
CollectionProxy
@association =
[#]
=> #
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repository.pull_requests
repository.pull_requests.to_a => []
repository.pull_requests.create
repository.pull_requests.to_a => [#]
HasManyAssociation
@loaded = true
@target = []
Relations
CollectionProxy
@association =
[#]
=> #
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Inverses /
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Inverses
class Repository < ApplicationRecord
has_many :pull_requests
end
class PullRequest < ApplicationRecord
belongs_to :repository
end
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Inverses
> pull_requests = repository.pull_requests
=> SELECT * FROM pull_requests WHERE repository_id = 42
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Inverses
> pull_requests = repository.pull_requests
=> SELECT * FROM pull_requests WHERE repository_id = 42
> pull_requests.map(&:repository)
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Inverses
> pull_requests = repository.pull_requests
=> SELECT * FROM pull_requests WHERE repository_id = 42
> pull_requests.map(&:repository)
=> SELECT * FROM repositories WHERE id = 42 LIMIT 1
=> SELECT * FROM repositories WHERE id = 42 LIMIT 1
=> SELECT * FROM repositories WHERE id = 42 LIMIT 1
=> SELECT * FROM repositories WHERE id = 42 LIMIT 1
=> SELECT * FROM repositories WHERE id = 42 LIMIT 1
0
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BelongsToAssociation
@owner = #
@loaded = false
@target = nil
Inverses
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BelongsToAssociation
@owner = #
@loaded = false
@target = nil
pull_request.repository
Inverses
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BelongsToAssociation
@owner = #
@loaded = false
@target = nil
pull_request.repository
Inverses
HasManyAssociation
@owner = #
@loaded = true
@target = [#, ...]
repository.pull_requests
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BelongsToAssociation
@owner = #
@loaded = false
@target = nil
pull_request.repository
Inverses
true
#
HasManyAssociation
@owner = #
@loaded = true
@target = [#, ...]
repository.pull_requests
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Inverses
HasManyAssociation
@owner = #
@loaded = true
@target = [#, ...]
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Inverses
@target.each do |record|
HasManyAssociation
@owner = #
@loaded = true
@target = [#, ...]
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Inverses
@target.each do |record| #
HasManyAssociation
@owner = #
@loaded = true
@target = [#, ...]
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Inverses
@target.each do |record|
association = record.association(:repository)
#
HasManyAssociation
@owner = #
@loaded = true
@target = [#, ...]
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Inverses
@target.each do |record|
association = record.association(:repository)
association.target = @owner
#
HasManyAssociation
@owner = #
@loaded = true
@target = [#, ...]
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Inverses
@target.each do |record|
association = record.association(:repository)
association.target = @owner #
#
HasManyAssociation
@owner = #
@loaded = true
@target = [#, ...]
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Inverses
@target.each do |record|
association = record.association(:repository)
association.target = @owner
end
Harcoded
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Reflection
@active_record = Repository
@name = :pull_requests
Inverses
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#inverse_name => :repository
Reflection
@active_record = Repository
@name = :pull_requests
Inverses
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#inverse_name => :repository
Reflection
@active_record = Repository
@name = :pull_requests
Inverses
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Inverses
class Reflection
def inverse_name
@active_record.name.underscore
end
end
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Inverses
@target.each do |record|
association = record.association(:repository)
association.target = @owner
end
Harcoded
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Inverses
@target.each do |record|
association = record.association(:repository)
association.target = @owner
end
Harcoded
@reflection.inverse_name)
Generic
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Inverses
> pull_requests = repository.pull_requests
=> SELECT * FROM pull_requests WHERE repository_id = 42
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Inverses
> pull_requests = repository.pull_requests
=> SELECT * FROM pull_requests WHERE repository_id = 42
> pull_requests.map(&:repository)
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Inverses
> pull_requests = repository.pull_requests
=> SELECT * FROM pull_requests WHERE repository_id = 42
> pull_requests.map(&:repository)
1
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2
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Further Studies
• Through Associations, Polymorphic Associations, Scoping, Etc.
• github.com/rails/rails
• activerecord/lib/active_record/associations.rb
• activerecord/lib/active_record/reflection.rb
• activerecord/lib/active_record/associations/*
• danieljamescolson.com/blog
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Practical Applications
• Use Rails features
• Write less custom code
• Write more efficient code
• Avoid inconsistent data
• Code with confidence
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Reflecting on
Active Record Associations
@composerinteralia
Daniel Colson
@dodecadaniel