The has_many Association in Java

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74 The has_many Association
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:inverse of => name of belongs to association
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Explicitly declares the name of the inverse association in a bidirectional relationship Considered an optimization, use of this option allows Rails to return the same instance of an object no matter which side of the relationship it is accessed from Consider the following, using our recurring example, without usage of inverse_of
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>> >> => >> => user = Userfirst timesheet = usertimesheetsfirst <Timesheet id: 1, user_id: 1> timesheetuserequal user false
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If we add :inverse_of to the association objection on User, like
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has_many :timesheets, :inverse_of => :user
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Active Record
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then timesheetuserequal user will be true Try something similar in one of your apps to see it for yourself Tim says
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With :inverse_of properly set, you ll still notice the following inconsistency:
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>> => >> => timesheetuserequal timesheetuser true userequal timesheetuser false
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This is because timesheetuser is actually an association proxy, and equal is quite the stickler for object identity
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>> userequal timesheetuserproxy_target => true
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:limit => integer
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Appends a LIMIT clause to the SQL generated for loading this association This option is potentially useful in capping the size of very large association collections Use in conjunction with the :order option to make sure your grabbing the most relevant records
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7: Active Record Associations
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:offset => integer
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An integer determining the offset from where the rows should be fetched when loading the association collection I assume this is here mostly for completeness, since it s hard to envision a valid use case
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Specifies the order in which the associated objects are returned via an ORDER BY sql fragment, such as "last_name, first_name DESC"
:primary key => column name
Specifies a surrogate key to use instead of the owning record s primary key, whose value should be used when querying to fill the association collection
:readonly => true
Sets all records in the association collection to read-only mode, which prevents saving them
:select => expression
By default, this is * as in SELECT * FROM, but can be changed if you, for example, want to add additional calculated columns or piggyback additional columns from joins onto the associated object as its loaded
:source and :source type
Used exclusively as additional options to assist in using has_many :through associations with polymorphic belongs_to Covered in detail later in this chapter
:table name => names
The :table_name option lets you override the table names (FROM clause) that will be used in SQL statements generated for loading the association
:through => association name
Creates an association collection via another association See the section in this chapter entitled has_many :through for more information
:uniq => true
Strips duplicate objects from the collection Sometimes useful in conjunction with has_many :through
75 Many-to-Many Relationships
:validate => false
In cases where the child records in the association collection would be automatically saved by Active Record, this option (true by default) dictates whether to ensure that they are valid If you always want to check the validity of associated records when saving the owning record, then use validates_associated :association_name
75 Many-to-Many Relationships
Associating persistent objects via a join table can be one of the trickier aspects of objectrelational mapping to implement correctly in a framework Rails has a couple of techniques that let you represent many-to-many relationships in your model We ll start with the older and simpler has_and_belongs_to_many and then cover the newer has_many :through
Active Record
751 has and belongs to many
Before proceeding with this section, I must clear my conscience by stating that has_and_belongs_to_many is practically obsolete in the minds of many Rails developers, including the authors of this book Use has_many :through instead and your life should be a lot easier The section is preserved in this edition almost exactly as it appeared in the first, because it contains good techniques that enlighten the reader about nuances of Active Record behavior The has_and_belongs_to_many method establishes a link between two associated Active Record models via an intermediate join table Unless the join table is explicitly specified as an option, Rails guesses its name by concatenating the table names of the joined classes, in alphabetical order and separated with an underscore For example, if I was using has_and_belongs_to_many (or habtm for short) to establish a relationship between Timesheet and BillingCode, the join table would be named billing_codes_timesheets and the relationship would be defined in the models Both the migration class and models are listed:
class CreateBillingCodesTimesheets < ActiveRecord::Migration def selfup create_table :billing_codes_timesheets, :id => false do |t| treferences :billing_code, :null => false treferences :timesheet, :null => false end end