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Diffstat (limited to 'guides/source/association_basics.md')
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1 files changed, 40 insertions, 13 deletions
diff --git a/guides/source/association_basics.md b/guides/source/association_basics.md index 884aa6a9ea..df38bd7321 100644 --- a/guides/source/association_basics.md +++ b/guides/source/association_basics.md @@ -40,7 +40,7 @@ end @customer.destroy ``` -With Active Record associations, we can streamline these — and other — operations by declaratively telling Rails that there is a connection between the two models. Here's the revised code for setting up customers and orders: +With Active Record associations, we can streamline these - and other - operations by declaratively telling Rails that there is a connection between the two models. Here's the revised code for setting up customers and orders: ```ruby class Customer < ActiveRecord::Base @@ -69,7 +69,7 @@ To learn more about the different types of associations, read the next section o The Types of Associations ------------------------- -In Rails, an _association_ is a connection between two Active Record models. Associations are implemented using macro-style calls, so that you can declaratively add features to your models. For example, by declaring that one model `belongs_to` another, you instruct Rails to maintain Primary Key–Foreign Key information between instances of the two models, and you also get a number of utility methods added to your model. Rails supports six types of associations: +In Rails, an _association_ is a connection between two Active Record models. Associations are implemented using macro-style calls, so that you can declaratively add features to your models. For example, by declaring that one model `belongs_to` another, you instruct Rails to maintain Primary Key-Foreign Key information between instances of the two models, and you also get a number of utility methods added to your model. Rails supports six types of associations: * `belongs_to` * `has_one` @@ -261,7 +261,10 @@ With `through: :sections` specified, Rails will now understand: ### The `has_one :through` Association -A `has_one :through` association sets up a one-to-one connection with another model. This association indicates that the declaring model can be matched with one instance of another model by proceeding _through_ a third model. For example, if each supplier has one account, and each account is associated with one account history, then the customer model could look like this: +A `has_one :through` association sets up a one-to-one connection with another model. This association indicates +that the declaring model can be matched with one instance of another model by proceeding _through_ a third model. +For example, if each supplier has one account, and each account is associated with one account history, then the +supplier model could look like this: ```ruby class Supplier < ActiveRecord::Base @@ -337,7 +340,7 @@ class CreateAssembliesAndParts < ActiveRecord::Migration t.timestamps end - create_table :assemblies_parts do |t| + create_table :assemblies_parts, id: false do |t| t.belongs_to :assembly t.belongs_to :part end @@ -487,6 +490,19 @@ end With this setup, you can retrieve `@employee.subordinates` and `@employee.manager`. +In your migrations/schema, you will add a references column to the model itself. + +```ruby +class CreateEmployees < ActiveRecord::Migration + def change + create_table :employees do |t| + t.references :manager + t.timestamps + end + end +end +``` + Tips, Tricks, and Warnings -------------------------- @@ -555,7 +571,7 @@ If you create an association some time after you build the underlying model, you If you create a `has_and_belongs_to_many` association, you need to explicitly create the joining table. Unless the name of the join table is explicitly specified by using the `:join_table` option, Active Record creates the name by using the lexical order of the class names. So a join between customer and order models will give the default join table name of "customers_orders" because "c" outranks "o" in lexical ordering. -WARNING: The precedence between model names is calculated using the `<` operator for `String`. This means that if the strings are of different lengths, and the strings are equal when compared up to the shortest length, then the longer string is considered of higher lexical precedence than the shorter one. For example, one would expect the tables "paper\_boxes" and "papers" to generate a join table name of "papers\_paper\_boxes" because of the length of the name "paper\_boxes", but it in fact generates a join table name of "paper\_boxes\_papers" (because the underscore '\_' is lexicographically _less_ than 's' in common encodings). +WARNING: The precedence between model names is calculated using the `<` operator for `String`. This means that if the strings are of different lengths, and the strings are equal when compared up to the shortest length, then the longer string is considered of higher lexical precedence than the shorter one. For example, one would expect the tables "paper_boxes" and "papers" to generate a join table name of "papers_paper_boxes" because of the length of the name "paper_boxes", but it in fact generates a join table name of "paper_boxes_papers" (because the underscore '_' is lexicographically _less_ than 's' in common encodings). Whatever the name, you must manually generate the join table with an appropriate migration. For example, consider these associations: @@ -715,7 +731,7 @@ The `belongs_to` association creates a one-to-one match with another model. In d #### Methods Added by `belongs_to` -When you declare a `belongs_to` association, the declaring class automatically gains four methods related to the association: +When you declare a `belongs_to` association, the declaring class automatically gains five methods related to the association: * `association(force_reload = false)` * `association=(associate)` @@ -861,8 +877,12 @@ end Counter cache columns are added to the containing model's list of read-only attributes through `attr_readonly`. ##### `:dependent` +If you set the `:dependent` option to: -If you set the `:dependent` option to `:destroy`, then deleting this object will call the `destroy` method on the associated object to delete that object. If you set the `:dependent` option to `:delete`, then deleting this object will delete the associated object _without_ calling its `destroy` method. If you set the `:dependent` option to `:restrict`, then attempting to delete this object will result in a `ActiveRecord::DeleteRestrictionError` if there are any associated objects. +* `:destroy`, when the object is destroyed, `destroy` will be called on its +associated objects. +* `:delete`, when the object is destroyed, all its associated objects will be +deleted directly from the database without calling their `destroy` method. WARNING: You should not specify this option on a `belongs_to` association that is connected with a `has_many` association on the other class. Doing so can lead to orphaned records in your database. @@ -953,7 +973,7 @@ end ##### `includes` -You can use the `includes` method let you specify second-order associations that should be eager-loaded when this association is used. For example, consider these models: +You can use the `includes` method to specify second-order associations that should be eager-loaded when this association is used. For example, consider these models: ```ruby class LineItem < ActiveRecord::Base @@ -1019,7 +1039,7 @@ The `has_one` association creates a one-to-one match with another model. In data #### Methods Added by `has_one` -When you declare a `has_one` association, the declaring class automatically gains four methods related to the association: +When you declare a `has_one` association, the declaring class automatically gains five methods related to the association: * `association(force_reload = false)` * `association=(associate)` @@ -1137,6 +1157,12 @@ Controls what happens to the associated object when its owner is destroyed: * `:restrict_with_exception` causes an exception to be raised if there is an associated record * `:restrict_with_error` causes an error to be added to the owner if there is an associated object +It's necessary not to set or leave `:nullify` option for those associations +that have `NOT NULL` database constraints. If you don't set `dependent` to +destroy such associations you won't be able to change the associated object +because initial associated object foreign key will be set to unallowed `NULL` +value. + ##### `:foreign_key` By convention, Rails assumes that the column used to hold the foreign key on the other model is the name of this model with the suffix `_id` added. The `:foreign_key` option lets you set the name of the foreign key directly: @@ -1280,7 +1306,7 @@ The `has_many` association creates a one-to-many relationship with another model #### Methods Added by `has_many` -When you declare a `has_many` association, the declaring class automatically gains 13 methods related to the association: +When you declare a `has_many` association, the declaring class automatically gains 16 methods related to the association: * `collection(force_reload = false)` * `collection<<(object, ...)` @@ -1769,7 +1795,7 @@ The `has_and_belongs_to_many` association creates a many-to-many relationship wi #### Methods Added by `has_and_belongs_to_many` -When you declare a `has_and_belongs_to_many` association, the declaring class automatically gains 13 methods related to the association: +When you declare a `has_and_belongs_to_many` association, the declaring class automatically gains 16 methods related to the association: * `collection(force_reload = false)` * `collection<<(object, ...)` @@ -1944,8 +1970,8 @@ While Rails uses intelligent defaults that will work well in most situations, th ```ruby class Parts < ActiveRecord::Base - has_and_belongs_to_many :assemblies, uniq: true, - read_only: true + has_and_belongs_to_many :assemblies, autosave: true, + readonly: true end ``` @@ -1957,6 +1983,7 @@ The `has_and_belongs_to_many` association supports these options: * `:foreign_key` * `:join_table` * `:validate` +* `:readonly` ##### `:association_foreign_key` |