More On Active Records

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More On Active Records
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Foreign Keys

• Active Records needs help with relationships
• One to one
• One to many
• Many to many
• The foreign keys are specified, by convention,
  using column names in the form of model name
  appended with _id
• For example graduate_id

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Specifying Relationships

• You can specify using
• has_one
• has_many
• belongs_to
• has_and_belongs_to_many
• These specify relationships and add useful
  methods
• belongs_to causes the object related to be saved
  appropriately
• depend gt true in has_one causes the related
  object to be deleted along with containing object

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Performance of Queries

• By default, of queries increases when you
  access objects with relationships
• One query to get object
• additional queries to get related objects
• Often called n1 problem
• You can hint Active Records to pre-fetch in the
  find method
• May not always improve performance
• DB Required to support left outer joints
• May bring large amount of unwanted data (in some
  cases) and unnecessary conversion to objects

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Transactions

• transaction method bounds operations
• Protects database
• But not object modified in memory

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Transactions

• You can send bound objects to transaction method
• Restores the objects if transaction fails

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Aggregation

• Allows you to map certain columns of your table
  as an object within your object
• composed_of attr_name, class_name gt ClassName,
  mapping gt mapping

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Inheritance

• OO world uses inheritance
• Relational relies on association
• Single table inheritance is an approach

type
id
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Validation

• Contents of object can be checked for validity
  during save
• Certain validators can be run on
• Create only
• Update only
• Save (create and update)
• validate
• validate_on_create
• validate_on_update
• You can explicitly validate by calling valid?()
  method
• errors holds any validation errors
• errors.on(symbol)
• errors.clear()

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Validation
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Helpers

• Above validators can be written using helpers
• Several of these validation helpers are provided

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Callback

• Active Records monitors objects through objects
  lifecycle
• It gives us opportunity to do special operations
• We will make good use of this to do things like
  encrypting of data before save
• You can provide these as methods or as handlers
• Except after_find and after_initialize which are
  required to be methods

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Callback
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Observers

• You may pollute your model with handlers
• Observe can make for a better cohesion