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Diffstat (limited to 'guides/source/active_record_querying.md')
| -rw-r--r-- | guides/source/active_record_querying.md | 87 |
1 files changed, 69 insertions, 18 deletions
diff --git a/guides/source/active_record_querying.md b/guides/source/active_record_querying.md index c4d69908ed..57e8e080f4 100644 --- a/guides/source/active_record_querying.md +++ b/guides/source/active_record_querying.md @@ -58,6 +58,7 @@ The methods are: * `bind` * `create_with` +* `distinct` * `eager_load` * `extending` * `from` @@ -76,7 +77,6 @@ The methods are: * `reorder` * `reverse_order` * `select` -* `distinct` * `uniq` * `where` @@ -91,7 +91,7 @@ The primary operation of `Model.find(options)` can be summarized as: ### Retrieving a Single Object -Active Record provides five different ways of retrieving a single object. +Active Record provides several different ways of retrieving a single object. #### Using a Primary Key @@ -524,12 +524,18 @@ To retrieve records from the database in a specific order, you can use the `orde For example, if you're getting a set of records and want to order them in ascending order by the `created_at` field in your table: ```ruby +Client.order(:created_at) +# OR Client.order("created_at") ``` You could specify `ASC` or `DESC` as well: ```ruby +Client.order(created_at: :desc) +# OR +Client.order(created_at: :asc) +# OR Client.order("created_at DESC") # OR Client.order("created_at ASC") @@ -538,16 +544,20 @@ Client.order("created_at ASC") Or ordering by multiple fields: ```ruby +Client.order(orders_count: :asc, created_at: :desc) +# OR +Client.order(:orders_count, created_at: :desc) +# OR Client.order("orders_count ASC, created_at DESC") # OR Client.order("orders_count ASC", "created_at DESC") ``` -If you want to call `order` multiple times e.g. in different context, new order will prepend previous one +If you want to call `order` multiple times e.g. in different context, new order will append previous one ```ruby Client.order("orders_count ASC").order("created_at DESC") -# SELECT * FROM clients ORDER BY created_at DESC, orders_count ASC +# SELECT * FROM clients ORDER BY orders_count ASC, created_at DESC ``` Selecting Specific Fields @@ -687,6 +697,10 @@ The SQL that would be executed: ```sql SELECT * FROM posts WHERE id > 10 LIMIT 20 + +# Original query without `except` +SELECT * FROM posts WHERE id > 10 ORDER BY id asc LIMIT 20 + ``` ### `unscope` @@ -707,7 +721,7 @@ Post.order('id DESC').limit(20).unscope(:order, :limit) = Post.all You can additionally unscope specific where clauses. For example: ```ruby -Post.where(:id => 10).limit(1).unscope(where: :id, :limit).order('id DESC') = Post.order('id DESC') +Post.where(id: 10).limit(1).unscope({ where: :id }, :limit).order('id DESC') = Post.order('id DESC') ``` ### `only` @@ -722,6 +736,10 @@ The SQL that would be executed: ```sql SELECT * FROM posts WHERE id > 10 ORDER BY id DESC + +# Original query without `only` +SELECT "posts".* FROM "posts" WHERE (id > 10) ORDER BY id desc LIMIT 20 + ``` ### `reorder` @@ -925,7 +943,7 @@ WARNING: This method only works with `INNER JOIN`. Active Record lets you use the names of the [associations](association_basics.html) defined on the model as a shortcut for specifying `JOIN` clause for those associations when using the `joins` method. -For example, consider the following `Category`, `Post`, `Comments` and `Guest` models: +For example, consider the following `Category`, `Post`, `Comment`, `Guest` and `Tag` models: ```ruby class Category < ActiveRecord::Base @@ -1171,7 +1189,7 @@ class Post < ActiveRecord::Base end ``` -This may then be called using this: +Call the scope as if it were a class method: ```ruby Post.created_before(Time.zone.now) @@ -1202,9 +1220,7 @@ class User < ActiveRecord::Base scope :active, -> { where state: 'active' } scope :inactive, -> { where state: 'inactive' } end -``` -```ruby User.active.inactive # => SELECT "users".* FROM "users" WHERE "users"."state" = 'active' AND "users"."state" = 'inactive' ``` @@ -1230,7 +1246,7 @@ One important caveat is that `default_scope` will be overridden by ```ruby class User < ActiveRecord::Base - default_scope { where state: 'pending' } + default_scope { where state: 'pending' } scope :active, -> { where state: 'active' } scope :inactive, -> { where state: 'inactive' } end @@ -1342,7 +1358,7 @@ COMMIT The new record might not be saved to the database; that depends on whether validations passed or not (just like `create`). -Suppose we want to set the 'locked' attribute to true if we're +Suppose we want to set the 'locked' attribute to `false` if we're creating a new record, but we don't want to include it in the query. So we want to find the client named "Andy", or if that client doesn't exist, create a client named "Andy" which is not locked. @@ -1450,7 +1466,7 @@ Client.pluck(:id, :name) # => [[1, 'David'], [2, 'Jeremy'], [3, 'Jose']] ``` -`pluck` makes it possible to replace code like +`pluck` makes it possible to replace code like: ```ruby Client.select(:id).map { |c| c.id } @@ -1460,7 +1476,7 @@ Client.select(:id).map(&:id) Client.select(:id, :name).map { |c| [c.id, c.name] } ``` -with +with: ```ruby Client.pluck(:id) @@ -1468,6 +1484,37 @@ Client.pluck(:id) Client.pluck(:id, :name) ``` +Unlike `select`, `pluck` directly converts a database result into a Ruby `Array`, +without constructing `ActiveRecord` objects. This can mean better performance for +a large or often-running query. However, any model method overrides will +not be available. For example: + +```ruby +class Client < ActiveRecord::Base + def name + "I am #{super}" + end +end + +Client.select(:name).map &:name +# => ["I am David", "I am Jeremy", "I am Jose"] + +Client.pluck(:name) +# => ["David", "Jeremy", "Jose"] +``` + +Furthermore, unlike `select` and other `Relation` scopes, `pluck` triggers an immediate +query, and thus cannot be chained with any further scopes, although it can work with +scopes already constructed earlier: + +```ruby +Client.pluck(:name).limit(1) +# => NoMethodError: undefined method `limit' for #<Array:0x007ff34d3ad6d8> + +Client.limit(1).pluck(:name) +# => ["David"] +``` + ### `ids` `ids` can be used to pluck all the IDs for the relation using the table's primary key. @@ -1489,18 +1536,21 @@ Person.ids Existence of Objects -------------------- -If you simply want to check for the existence of the object there's a method called `exists?`. This method will query the database using the same query as `find`, but instead of returning an object or collection of objects it will return either `true` or `false`. +If you simply want to check for the existence of the object there's a method called `exists?`. +This method will query the database using the same query as `find`, but instead of returning an +object or collection of objects it will return either `true` or `false`. ```ruby Client.exists?(1) ``` -The `exists?` method also takes multiple ids, but the catch is that it will return true if any one of those records exists. +The `exists?` method also takes multiple values, but the catch is that it will return `true` if any +one of those records exists. ```ruby -Client.exists?(1,2,3) +Client.exists?(id: [1,2,3]) # or -Client.exists?([1,2,3]) +Client.exists?(name: ['John', 'Sergei']) ``` It's even possible to use `exists?` without any arguments on a model or a relation. @@ -1509,7 +1559,8 @@ It's even possible to use `exists?` without any arguments on a model or a relati Client.where(first_name: 'Ryan').exists? ``` -The above returns `true` if there is at least one client with the `first_name` 'Ryan' and `false` otherwise. +The above returns `true` if there is at least one client with the `first_name` 'Ryan' and `false` +otherwise. ```ruby Client.exists? |