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 several finder methods. Each finder method allows you to pass arguments into it to perform certain queries on your database without writing raw SQL.
Using <tt>Model.find(primary_key)</tt>, you can retrieve the object corresponding to the supplied _primary key_ and matching the supplied options (if any). For example:
<tt>Model.find(array_of_primary_key)</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.
This is because +User.all.each+ makes Active Record fetch _the entire table_, build a model object per row, and keep the entire array in the memory. Sometimes that is just too many objects and demands too much memory.
To efficiently iterate over a large table, Active Record provides a batch finder method called +find_each+:
<ruby>
User.find_each do |user|
NewsLetter.weekly_deliver(user)
end
</ruby>
*Configuring the batch size*
Behind the scenes +find_each+ fetches rows in batches of +1000+ and yields them one by one. The size of the underlying batches is configurable via the +:batch_size+ option.
To fetch +User+ records in batch size of +5000+:
<ruby>
User.find_each(:batch_size => 5000) do |user|
NewsLetter.weekly_deliver(user)
end
</ruby>
*Starting batch find from a specific primary key*
Records are fetched in ascending order on the primary key, which must be an integer. The +:start+ option allows you to configure the first ID of the sequence if the lowest is not the one you need. This may be useful for example to be able to resume an interrupted batch process if it saves the last processed ID as a checkpoint.
To send newsletters only to users with the primary key starting from +2000+:
<ruby>
User.find_each(:batch_size => 5000, :start => 2000) do |user|
+find_each+ accepts the same options as the regular +find+ method. However, +:order+ and +:limit+ are needed internally and hence not allowed to be passed explicitly.
You can also work by chunks instead of row by row using +find_in_batches+. This method is analogous to +find_each+, but it yields arrays of models instead:
<ruby>
# Works in chunks of 1000 invoices at a time.
Invoice.find_in_batches(:include => :invoice_lines) do |invoices|
export.add_invoices(invoices)
end
</ruby>
The above will yield the supplied block with +1000+ invoices every time.
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.where("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.where("first_name LIKE '%#{params[:first_name]}%'")+ is not safe. See the next section for the preferred way to handle conditions using an array.
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.
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:
Active Record also allows you to pass in 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:
You can use +limit+ to specify the number of records to be retrieved, and use +offset+ to specify the number of records to skip before starting to return the records. For example
SQL uses the +HAVING+ clause to specify conditions on the +GROUP BY+ fields. You can add the +HAVING+ clause to the SQL fired by the +Model.find+ by adding the +:having+ option to the find.
Active Record provides +readonly+ method on a relation to explicitly disallow modification or deletion of any of the returned object. Any attempt to alter or destroy a readonly record will not succeed, raising an +ActiveRecord::ReadOnlyRecord+ exception.
As +client+ is explicitly set to be a readonly object, the above code will raise an +ActiveRecord::ReadOnlyRecord+ exception when calling +client.save+ with an updated value of _visists_.
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.
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. If an update request is made with a lower value in the +lock_version+ field than is currently in the +lock_version+ column in the database, the update request will fail with an +ActiveRecord::StaleObjectError+. Example:
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 a locking mechanism provided by the underlying database. Using +lock+ when building a relation obtains an exclusive lock on the selected rows. Relations using +lock+ are usually wrapped inside a transaction for preventing deadlock conditions.
You can also pass raw SQL to the +lock+ method for allowing 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:
Active Record provides a finder method called +joins+ for specifying +JOIN+ clauses on the resulting SQL. There are multiple ways to use the +joins+ method.
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.
You can specify conditions on the joined tables using the regular "Array":#array-conditions and "String":#pure-string-conditions conditions. "Hash conditions":#hash-conditions 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 in advance all the associations that are going to be loaded. This is possible by specifying the +includes+ method of the +Model.find+ call. With +includes+, Active Record ensures that all of the specified associations are loaded using the minimum possible number of queries.
Active Record lets you eager load any number of associations with a single +Model.find+ call by using an array, hash, or a nested hash of array/hash with the +includes+ method.
This will find the category with id 1 and eager load all of the associated posts, the associated posts' tags and comments, and every comment's guest association.
Even though Active Record lets you specify conditions on the eager loaded associations just like +joins+, the recommended way is to use "joins":#joining-tables 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 +first_name+ on your +Client+ model for example, you get +find_by_first_name+ and +find_all_by_first_name+ for free from Active Record. If you have a +locked+ field on the +Client+ model, you also get +find_by_locked+ and +find_all_by_locked+ methods.
You can specify an exclamation point (<tt>!</tt>) 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_first_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_first_name(params[:first_name])+. Using this will first perform a find and then create if the find returns +nil+. The SQL looks like this for +Client.find_or_create_by_first_name("Ryan")+:
+find_or_create+'s sibling, +find_or_initialize+, will find an object and if it does not exist will act similarly 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(:first_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.
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 if 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>
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.
