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Merge branch 'master' of github.com:rails/docrails

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	activerecord/lib/active_record/base.rb
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Vijay Dev 2014-07-12 10:24:49 +00:00
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236
guides/source/active_record_querying.md
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@ -93,9 +93,9 @@ The primary operation of `Model.find(options)` can be summarized as:
Active Record provides several different ways of retrieving a single object.
#### Using a Primary Key
#### `find`
Using `Model.find(primary_key)`, you can retrieve the object corresponding to the specified _primary key_ that matches any supplied options. For example:
Using the `find` method, you can retrieve the object corresponding to the specified _primary key_ that matches any supplied options. For example:
```ruby
# Find the client with primary key (id) 10.
@ -109,11 +109,27 @@ The SQL equivalent of the above is:
SELECT * FROM clients WHERE (clients.id = 10) LIMIT 1
```
`Model.find(primary_key)` will raise an `ActiveRecord::RecordNotFound` exception if no matching record is found.
The `find` method will raise an `ActiveRecord::RecordNotFound` exception if no matching record is found.
You can also use this method to query for multiple objects. Call the `find` method and pass in an array of primary keys. The return will be an array containing all of the matching records for the supplied _primary keys_. For example:
```ruby
# Find the clients with primary keys 1 and 10.
client = Client.find([1, 10]) # Or even Client.find(1, 10)
# => [#<Client id: 1, first_name: "Lifo">, #<Client id: 10, first_name: "Ryan">]
```
The SQL equivalent of the above is:
```sql
SELECT * FROM clients WHERE (clients.id IN (1,10))
```
WARNING: The `find` method will raise an `ActiveRecord::RecordNotFound` exception unless a matching record is found for **all** of the supplied primary keys.
#### `take`
`Model.take` retrieves a record without any implicit ordering. For example:
The `take` method retrieves a record without any implicit ordering. For example:
```ruby
client = Client.take
@ -126,13 +142,31 @@ The SQL equivalent of the above is:
SELECT * FROM clients LIMIT 1
```
`Model.take` returns `nil` if no record is found and no exception will be raised.
The `take` method returns `nil` if no record is found and no exception will be raised.
You can pass in a numerical argument to the `take` method to return up to that number of results. For example
```ruby
client = Client.take(2)
# => [
#<Client id: 1, first_name: "Lifo">,
#<Client id: 220, first_name: "Sara">
]
```
The SQL equivalent of the above is:
```sql
SELECT * FROM clients LIMIT 2
```
The `take!` method behaves exactly like `take`, except that it will raise `ActiveRecord::RecordNotFound` if no matching record is found.
TIP: The retrieved record may vary depending on the database engine.
#### `first`
`Model.first` finds the first record ordered by the primary key. For example:
The `first` method finds the first record ordered by the primary key. For example:
```ruby
client = Client.first
@ -145,11 +179,30 @@ The SQL equivalent of the above is:
SELECT * FROM clients ORDER BY clients.id ASC LIMIT 1
```
`Model.first` returns `nil` if no matching record is found and no exception will be raised.
The `first` method returns `nil` if no matching record is found and no exception will be raised.
You can pass in a numerical argument to the `first` method to return up to that number of results. For example
```ruby
client = Client.first(3)
# => [
#<Client id: 1, first_name: "Lifo">,
#<Client id: 2, first_name: "Fifo">,
#<Client id: 3, first_name: "Filo">
]
```
The SQL equivalent of the above is:
```sql
SELECT * FROM clients ORDER BY clients.id ASC LIMIT 3
```
The `first!` method behaves exactly like `first`, except that it will raise `ActiveRecord::RecordNotFound` if no matching record is found.
#### `last`
`Model.last` finds the last record ordered by the primary key. For example:
The `last` method finds the last record ordered by the primary key. For example:
```ruby
client = Client.last
@ -162,11 +215,30 @@ The SQL equivalent of the above is:
SELECT * FROM clients ORDER BY clients.id DESC LIMIT 1
```
`Model.last` returns `nil` if no matching record is found and no exception will be raised.
The `last` method returns `nil` if no matching record is found and no exception will be raised.
You can pass in a numerical argument to the `last` method to return up to that number of results. For example
```ruby
client = Client.last(3)
# => [
#<Client id: 219, first_name: "James">,
#<Client id: 220, first_name: "Sara">,
#<Client id: 221, first_name: "Russel">
]
```
The SQL equivalent of the above is:
```sql
SELECT * FROM clients ORDER BY clients.id DESC LIMIT 3
```
The `last!` method behaves exactly like `last`, except that it will raise `ActiveRecord::RecordNotFound` if no matching record is found.
#### `find_by`
`Model.find_by` finds the first record matching some conditions. For example:
The `find_by` method finds the first record matching some conditions. For example:
```ruby
Client.find_by first_name: 'Lifo'
@ -182,40 +254,19 @@ It is equivalent to writing:
Client.where(first_name: 'Lifo').take
```
#### `take!`
`Model.take!` retrieves a record without any implicit ordering. For example:
The `find_by!` method behaves exactly like `find_by`, except that it will raise `ActiveRecord::RecordNotFound` if no matching record is found. For example:
```ruby
client = Client.take!
# => #<Client id: 1, first_name: "Lifo">
Client.find_by! first_name: 'does not exist'
# => ActiveRecord::RecordNotFound
```
The SQL equivalent of the above is:
```sql
SELECT * FROM clients LIMIT 1
```
`Model.take!` raises `ActiveRecord::RecordNotFound` if no matching record is found.
#### `first!`
`Model.first!` finds the first record ordered by the primary key. For example:
This is equivalent to writing:
```ruby
client = Client.first!
# => #<Client id: 1, first_name: "Lifo">
Client.where(first_name: 'does not exist').take!
```
The SQL equivalent of the above is:
```sql
SELECT * FROM clients ORDER BY clients.id ASC LIMIT 1
```
`Model.first!` raises `ActiveRecord::RecordNotFound` if no matching record is found.
#### `last!`
`Model.last!` finds the last record ordered by the primary key. For example:
@ -233,92 +284,6 @@ SELECT * FROM clients ORDER BY clients.id DESC LIMIT 1
`Model.last!` raises `ActiveRecord::RecordNotFound` if no matching record is found.
#### `find_by!`
`Model.find_by!` finds the first record matching some conditions. It raises `ActiveRecord::RecordNotFound` if no matching record is found. For example:
```ruby
Client.find_by! first_name: 'Lifo'
# => #<Client id: 1, first_name: "Lifo">
Client.find_by! first_name: 'Jon'
# => ActiveRecord::RecordNotFound
```
It is equivalent to writing:
```ruby
Client.where(first_name: 'Lifo').take!
```
### Retrieving Multiple Objects
#### Using Multiple Primary Keys
`Model.find(array_of_primary_key)` accepts an array of _primary keys_, returning an array containing all of the matching records for the supplied _primary keys_. For example:
```ruby
# Find the clients with primary keys 1 and 10.
client = Client.find([1, 10]) # Or even Client.find(1, 10)
# => [#<Client id: 1, first_name: "Lifo">, #<Client id: 10, first_name: "Ryan">]
```
The SQL equivalent of the above is:
```sql
SELECT * FROM clients WHERE (clients.id IN (1,10))
```
WARNING: `Model.find(array_of_primary_key)` will raise an `ActiveRecord::RecordNotFound` exception unless a matching record is found for **all** of the supplied primary keys.
#### take
`Model.take(limit)` retrieves the first number of records specified by `limit` without any explicit ordering:
```ruby
Client.take(2)
# => [#<Client id: 1, first_name: "Lifo">,
#<Client id: 2, first_name: "Raf">]
```
The SQL equivalent of the above is:
```sql
SELECT * FROM clients LIMIT 2
```
#### first
`Model.first(limit)` finds the first number of records specified by `limit` ordered by primary key:
```ruby
Client.first(2)
# => [#<Client id: 1, first_name: "Lifo">,
#<Client id: 2, first_name: "Raf">]
```
The SQL equivalent of the above is:
```sql
SELECT * FROM clients ORDER BY id ASC LIMIT 2
```
#### last
`Model.last(limit)` finds the number of records specified by `limit` ordered by primary key in descending order:
```ruby
Client.last(2)
# => [#<Client id: 10, first_name: "Ryan">,
#<Client id: 9, first_name: "John">]
```
The SQL equivalent of the above is:
```sql
SELECT * FROM clients ORDER BY id DESC LIMIT 2
```
### Retrieving Multiple Objects in Batches
We often need to iterate over a large set of records, as when we send a newsletter to a large set of users, or when we export data.
@ -344,7 +309,15 @@ The `find_each` method retrieves a batch of records and then yields _each_ recor
```ruby
User.find_each do |user|
NewsLetter.weekly_deliver(user)
NewsMailer.weekly(user).deliver
end
```
To add conditions to a `find_each` operation you can chain other Active Record methods such as `where`:
```ruby
User.where(weekly_subscriber: true).find_each do |user|
NewsMailer.weekly(user).deliver
end
```
@ -707,7 +680,7 @@ Overriding Conditions
You can specify certain conditions to be removed using the `unscope` method. For example:
```ruby
Article.where('id > 10').limit(20).order('id asc').except(:order)
Article.where('id > 10').limit(20).order('id asc').unscope(:order)
```
The SQL that would be executed:
@ -720,7 +693,7 @@ SELECT * FROM articles WHERE id > 10 ORDER BY id asc LIMIT 20
```
You can additionally unscope specific where clauses. For example:
You can also unscope specific `where` clauses. For example:
```ruby
Article.where(id: 10, trashed: false).unscope(where: :id)
@ -759,8 +732,6 @@ The `reorder` method overrides the default scope order. For example:
```ruby
class Article < ActiveRecord::Base
..
..
has_many :comments, -> { order('posted_at DESC') }
end
@ -1487,6 +1458,11 @@ If you'd like to use your own SQL to find records in a table you can use `find_b
Client.find_by_sql("SELECT * FROM clients
INNER JOIN orders ON clients.id = orders.client_id
ORDER BY clients.created_at desc")
# => [
#<Client id: 1, first_name: "Lucas" >,
#<Client id: 2, first_name: "Jan" >,
# ...
]
```
`find_by_sql` provides you with a simple way of making custom calls to the database and retrieving instantiated objects.
@ -1496,7 +1472,11 @@ Client.find_by_sql("SELECT * FROM clients
`find_by_sql` has a close relative called `connection#select_all`. `select_all` will retrieve objects from the database using custom SQL just like `find_by_sql` but will not instantiate them. Instead, you will get an array of hashes where each hash indicates a record.
```ruby
Client.connection.select_all("SELECT * FROM clients WHERE id = '1'")
Client.connection.select_all("SELECT first_name, created_at FROM clients WHERE id = '1'")
# => [
{"first_name"=>"Rafael", "created_at"=>"2012-11-10 23:23:45.281189"},
{"first_name"=>"Eileen", "created_at"=>"2013-12-09 11:22:35.221282"}
]
```
### `pluck`