Rails checking if a record exists in database

If you want to check for the existence of an object why not use exists?

if Truck.exists?(10)  # your truck exists in the databaseelse  # the truck doesn't existend

The exists? method has the advantage that is not selecting the record from the database (meaning is faster than selecting the record).The query looks like:

SELECT 1 FROM trucks where trucks.id = 10

You can find more examples in the Rails documentation for #exists?.

Here is how you can check this.

if Trucks.where(:id => current_truck.id).blank?  # no truck record for this idelse  # at least 1 record for this truckend

where method returns an ActiveRecord::Relation object (acts like an array which contains the results of the where), it can be empty but never be nil.

OP actual use case solution

The simplest solution is to combine your DB check and retrieval of data into 1 DB query instead of having separate DB calls. Your sample code is close and conveys your intent, but it's a little off in your actual syntax.

If you simple do Truck.where("id = ?", id).select('truck_no').first.truck_no and this record does NOT exists, it will throw a nil error when you call truck_no because first may retrieve a nil record if none are found that match your criteria.

That's because your query will return an array of objects that match your criteria, then you do a first on that array which (if no matching records are found) is nil.

A fairly clean solution:

# Note: using Rails 4 / Ruby 2 syntaxfirst_truck = Truck.select(:truck_no).find_by(id) # => <Truck id: nil, truck_no: "123"> OR nil if no record matches criteriaif first_truck  truck_number = first_truck.truck_no  # do some processing...else  # record does not exist with that criteriaend

I recommend using clean syntax that "comments" itself so others know exactly what you're trying to do.

If you really want to go the extra mile, you could add a method to your Truck class that does this for you and conveys your intent:

# truck.rb modelclass Truck < ActiveRecord::Base  def self.truck_number_if_exists(record_id)    record = Truck.select(:truck_no).find_by(record_id)    if record      record.truck_no    else      nil # explicit nil so other developers know exactly what's going on    end  endend

Then you would call it like so:

if truck_number = Truck.truck_number_if_exists(id)  # do processing because record exists and you have the valueelse  # no matching criteriaend

The ActiveRecord.find_by method will retrieve the first record that matches your criteria or else returns nil if no record is found with that criteria. Note that the order of the find_by and where methods is important; you must call the select on the Truck model. This is because when you call the where method you're actually returning an ActiveRelation object which is not what you're looking for here.

See ActiveRecord API for 'find_by' method

General solutions using 'exists?' method

As some of the other contributors have already mentioned, the exists? method is engineered specifically to check for the existence of something. It doesn't return the value, just confirms that the DB has a record that matches some criteria.

It is useful if you need to verify uniqueness or accuracy of some piece of data. The nice part is that it allows you to use the ActiveRelation(Record?) where(...) criteria.

For instance, if you have a User model with an email attribute and you need to check if an email already exists in the dB:

User.exists?(email: "test@test.com")

The benefit of using exists? is that the SQL query run is

SELECT 1 AS one FROM "users" WHERE "users"."email" = 'test@test.com' LIMIT 1

which is more efficient than actually returning data.

If you need to actually conditionally retrieve data from the DB this isn't the method to use. However, it works great for simple checking and the syntax is very clear so other developers know exactly what you're doing. Using appropriate syntax is critical in projects with multiple developers. Write clean code and let the code "comment" itself.