This guide covers different ways to retrieve data from the database using Active Record. By referring to this guide, you will be able to:
* Find records using a variety of methods and conditions
* Specify the order, retrieved attributes, grouping, and other properties of the found records
* Use eager loading to reduce the number of database queries needed for data retrieval
* Use dynamic finders methods
* Create named scopes to add custom finding behavior to your models
* Check for the existence of particular records
* Perform various calculations on Active Record models
endprologue.
If you're used to using raw SQL to find database records then, generally, you will find that there are better ways to carry out the same operations in Rails. Active Record insulates you from the need to use SQL in most cases.
Code examples throughout this guide will refer to one or more of the following models:
Active Record will perform queries on the database for you and is compatible with most database systems (MySQL, PostgreSQL and SQLite to name a few). Regardless of which database system you're using, the Active Record method format will always be the same.
To retrieve objects from the database, Active Record provides a class method called +Model.find+. This method allows you to pass arguments into it to perform certain queries on your database without the need of writing raw SQL.
Using <tt>Model.find(primary_key, options = nil)</tt>, you can retrieve the object corresponding to the supplied _primary key_ and matching the supplied options (if any). For example:
<tt>Model.first(options = nil)</tt> finds the first record matched by the supplied options. If no +options+ are supplied, the first matching record is returned. For example:
<tt>Model.last(options = nil)</tt> finds the last record matched by the supplied options. If no +options+ are supplied, the last matching record is returned. For example:
<tt>Model.find(array_of_primary_key, options = nil)</tt> also accepts an array of _primary keys_. An array of all the matching records for the supplied _primary keys_ is returned. For example:
<tt>Model.find(array_of_primary_key)</tt> will raise an +ActiveRecord::RecordNotFound+ exception unless a matching record is found for <strong>all</strong> of the supplied primary keys.
<tt>Model.all(options = nil)</tt> finds all the records matching the supplied +options+. If no +options+ are supplied, all rows from the database are returned.
The +find+ method allows you to specify conditions to limit the records returned, representing the WHERE-part of the SQL statement. Conditions can either be specified as a string, array, or hash.
If you'd like to add conditions to your find, you could just specify them in there, just like +Client.first(:conditions => "orders_count = '2'")+. This will find all clients where the +orders_count+ field's value is 2.
WARNING: Building your own conditions as pure strings can leave you vulnerable to SQL injection exploits. For example, +Client.first(:conditions => "name LIKE '%#{params[:name]}%'")+ is not safe. See the next section for the preferred way to handle conditions using an array.
Now what if that number could vary, say as a argument from somewhere, or perhaps from the user's level status somewhere? The find then becomes something like:
Active Record will go through the first element in the conditions value and any additional elements will replace the question marks +(?)+ in the first element.
In this example, the first question mark will be replaced with the value in +params[:orders]+ and the second will be replaced with the SQL representation of +false+, which depends on the adapter.
is because of argument safety. Putting the variable directly into the conditions string will pass the variable to the database *as-is*. This means that it will be an unescaped variable directly from a user who may have malicious intent. If you do this, you put your entire database at risk because once a user finds out he or she can exploit your database they can do just about anything to it. Never ever put your arguments directly inside the conditions string.
TIP: For more information on the dangers of SQL injection, see the "Ruby on Rails Security Guide":../security.html#_sql_injection.
If you're looking for a range inside of a table (for example, users created in a certain timeframe) you can use the conditions option coupled with the +IN+ SQL statement for this. If you had two dates coming in from a controller you could do something like this to look for a range:
This would generate the proper query which is great for small ranges but not so good for larger ranges. For example if you pass in a range of date objects spanning a year that's 365 (or possibly 366, depending on the year) strings it will attempt to match your field against.
Active Record also allows you to pass in a hash conditions which can increase the readability of your conditions syntax. With hash conditions, you pass in a hash with keys of the fields you want conditionalised and the values of how you want to conditionalise them:
Be careful because this also means you're initializing a model object with only the fields that you've selected. If you attempt to access a field that is not in the initialized record you'll receive:
Where +<attribute>+ is the attribute you asked for. The +id+ method will not raise the +ActiveRecord::MissingAttributeError+, so just be careful when working with associations because they need the +id+ method to function properly.
You can also call SQL functions within the select option. For example, if you would like to only grab a single record per unique value in a certain field by using the +DISTINCT+ function you can do it like this:
If you want to limit the amount of records to a certain subset of all the records retrieved you usually use +:limit+ for this, sometimes coupled with +:offset+. Limit is the maximum number of records that will be retrieved from a query, and offset is the number of records it will start reading from from the first record of the set. For example:
This code will return a maximum of 5 clients and because it specifies no offset it will return the first 5 clients in the table. The SQL it executes will look like this:
This code will return a maximum of 5 clients and because it specifies an offset this time, it will return these records starting from the 5th client in the clients table. The SQL looks like:
SQL uses +HAVING+ clause to specify conditions on the +GROUP BY+ fields. You can specify the +HAVING+ clause to the SQL fired by the +Model.find+ using +:having+ option on the find.
To explicitly disallow modification/destroyal of the matching records returned by +Model.find+, you could specify the +:readonly+ option as +true+ to the find call.
Any attempt to alter or destroy the readonly records will not succeed, raising an +ActiveRecord::ReadOnlyRecord+ exception. To set this option, specify it like this:
Locking is helpful for preventing the race conditions when updating records in the database and ensuring atomic updated. Active Record provides two locking mechanism:
Optimistic locking allows multiple users to access the same record for edits, and assumes a minimum of conflicts with the data. It does this by checking whether another process has made changes to a record since it was opened. An +ActiveRecord::StaleObjectError+ exception is thrown if that has occurred and the update is ignored.
<strong>Optimistic locking column</strong>
In order to use optimistic locking, the table needs to have a column called +lock_version+. Each time the record is updated, Active Record increments the +lock_version+ column and the locking facilities ensure that records instantiated twice will let the last one saved raise an +ActiveRecord::StaleObjectError+ exception if the first was also updated. Example:
<ruby>
c1 = Client.find(1)
c2 = Client.find(1)
c1.name = "Michael"
c1.save
c2.name = "should fail"
c2.save # Raises a ActiveRecord::StaleObjectError
</ruby>
You're then responsible for dealing with the conflict by rescuing the exception and either rolling back, merging, or otherwise apply the business logic needed to resolve the conflict.
NOTE: You must ensure that your database schema defaults the +lock_version+ column to +0+.
<br />
This behavior can be turned off by setting <tt>ActiveRecord::Base.lock_optimistically = false</tt>.
Pessimistic locking uses locking mechanism provided by the underlying database. Passing +:lock => true+ to +Model.find+ obtains an exclusive lock on the selected rows. +Model.find+ using +:lock+ are usually wrapped inside a transaction for preventing deadlock conditions.
The above session produces the following SQL for a MySQL backend:
<sql>
SQL (0.2ms) BEGIN
Item Load (0.3ms) SELECT * FROM `items` LIMIT 1 FOR UPDATE
Item Update (0.4ms) UPDATE `items` SET `updated_at` = '2009-02-07 18:05:56', `name` = 'Jones' WHERE `id` = 1
SQL (0.8ms) COMMIT
</sql>
You can also pass raw SQL to the +:lock+ option to allow different types of locks. For example, MySQL has an expression called +LOCK IN SHARE MODE+ where you can lock a record but still allow other queries to read it. To specify this expression just pass it in as the lock option:
<tt>Model.find</tt> provides a +:joins+ option for specifying +JOIN+ clauses on the resulting SQL. There multiple different ways to specify the +:joins+ option:
h4. Using a string SQL fragment
You can just supply the raw SQL specifying the +JOIN+ clause to the +:joins+ option. For example:
<ruby>
Client.all(:joins => 'LEFT OUTER JOIN addresses ON addresses.client_id = client.id')
</ruby>
This will result in the following SQL:
<sql>
SELECT clients.* FROM clients INNER JOIN addresses ON addresses.client_id = clients.id
</sql>
h4. Using Array/Hash of named associations
WARNING: This method only works with +INNER JOIN+,
<br />
Active Record lets you use the names of the "associations":association_basics.html defined on the Model, as a shortcut for specifying the +:joins+ option.
For example, consider the following +Category+, +Post+, +Comments+ and +Guest+ models:
<ruby>
class Category < ActiveRecord::Base
has_many :posts
end
class Post < ActiveRecord::Base
belongs_to :category
has_many :comments
has_many :tags
end
class Comments < ActiveRecord::Base
belongs_to :post
has_one :guest
end
class Guest < ActiveRecord::Base
belongs_to :comment
end
</ruby>
Now all of the following will produce the expected join queries using +INNER JOIN+:
h5. Joining a single association
<ruby>
Category.all :joins => :posts
</ruby>
This produces:
<sql>
SELECT categories.* FROM categories
INNER JOIN posts ON posts.category_id = categories.id
You can specify conditions on the joined tables using the regular "Array":#arrayconditions and "String":#purestringconditions conditions. "Hash conditions":#hashconditions provides a special syntax for specifying conditions for the joined tables:
Consider the following code, which finds 10 clients and prints their postcodes:
<ruby>
clients = Client.all(:limit => 10)
clients.each do |client|
puts client.address.postcode
end
</ruby>
This code looks fine at the first sight. But the problem lies within the total number of queries executed. The above code executes 1 ( to find 10 clients ) <plus> 10 ( one per each client to load the address ) = <strong>11</strong> queries in total.
<strong>Solution to N <plus> 1 queries problem</strong>
Active Record lets you specify all the associations in advanced that are going to be loaded. This is possible by specifying the +:include+ option of the +Model.find+ call. By +:include+, Active Record ensures that all the specified associations are loaded using minimum possible number of queries.
Revisiting the above case, we could rewrite +Client.all+ to use eager load addresses:
Active Record lets you eager load any possible number of associations with a single +Model.find+ call by using Array, Hash or a nested Hash of Array/Hash with +:include+ find option.
The above code finds the category with id 1 and eager loads all the posts associated with the found category. Additionally, it will also eager load every posts' tags and comments. Every comment's guest association will get eager loaded as well.
h4. Specifying conditions on eager loaded associations
Even though Active Record lets you specify conditions on the eager loaded associations just like +:joins+, the recommended way is to use ":joins":#joiningtables instead.
For every field (also known as an attribute) you define in your table, Active Record provides a finder method. If you have a field called +name+ on your Client model for example, you get +find_by_name+ and +find_all_by_name+ for free from Active Record. If you have also have a +locked+ field on the Client model, you also get +find_by_locked+ and +find_all_by_locked+.
You can do +find_last_by_*+ methods too which will find the last record matching your argument.
You can specify an exclamation point (!) on the end of the dynamic finders to get them to raise an ActiveRecord::RecordNotFound error if they do not return any records, like +Client.find_by_name!("Ryan")+
If you want to find both by name and locked, you can chain these finders together by simply typing +and+ between the fields for example +Client.find_by_name_and_locked("Ryan", true)+.
There's another set of dynamic finders that let you find or create/initialize objects if they aren't found. These work in a similar fashion to the other finders and can be used like +find_or_create_by_name(params[:name])+. Using this will firstly perform a find and then create if the find returns nil. The SQL looks like this for +Client.find_or_create_by_name("Ryan")+:
<sql>
SELECT * FROM clients WHERE (clients.name = 'Ryan') LIMIT 1
BEGIN
INSERT INTO clients (name, updated_at, created_at, orders_count, locked)
+find_or_create+'s sibling, +find_or_initialize+, will find an object and if it does not exist will act similar to calling +new+ with the arguments you passed in. For example:
will either assign an existing client object with the name 'Ryan' to the client local variable, or initialize a new object similar to calling +Client.new(:name => 'Ryan')+. From here, you can modify other fields in client by calling the attribute setters on it: +client.locked = true+ and when you want to write it to the database just call +save+ on it.
h3. Finding By SQL
If you'd like to use your own SQL to find records in a table you can use +find_by_sql+. The +find_by_sql+ method will return an array of objects even the underlying query returns just a single record. For example you could run this query:
<tt>find_by_sql</tt> 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.
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+.
SELECT count(DISTINCT clients.id) AS count_all FROM clients
LEFT OUTER JOIN orders ON orders.client_id = client.id WHERE
(clients.first_name = 'Ryan' AND orders.status = 'received')
</sql>
This code specifies +clients.first_name+ just in case one of the join tables has a field also called +first_name+ and it uses +orders.status+ because that's the name of our join table.
h4. Count
If you want to see how many records are in your model's table you could call +Client.count+ and that will return the number. If you want to be more specific and find all the clients with their age present in the database you can use +Client.count(:age)+.
For options, please see the parent section, "Calculations":#calculations.
h4. Average
If you want to see the average of a certain number in one of your tables you can call the +average+ method on the class that relates to the table. This method call will look something like this:
<ruby>
Client.average("orders_count")
</ruby>
This will return a number (possibly a floating point number such as 3.14159265) representing the average value in the field.
For options, please see the parent section, "Calculations":#calculations.
h4. Minimum
If you want to find the minimum value of a field in your table you can call the +minimum+ method on the class that relates to the table. This method call will look something like this:
<ruby>
Client.minimum("age")
</ruby>
For options, please see the parent section, "Calculations":#calculations.
h4. Maximum
If you want to find the maximum value of a field in your table you can call the +maximum+ method on the class that relates to the table. This method call will look something like this:
<ruby>
Client.maximum("age")
</ruby>
For options, please see the parent section, "Calculations":#calculations.
h4. Sum
If you want to find the sum of a field for all records in your table you can call the +sum+ method on the class that relates to the table. This method call will look something like this:
<ruby>
Client.sum("orders_count")
</ruby>
For options, please see the parent section, "Calculations":#calculations.
