Skip to content


Switch branches/tags

Name already in use

A tag already exists with the provided branch name. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. Are you sure you want to create this branch?

Latest commit


Git stats


Failed to load latest commit information.
Latest commit message
Commit time

What to do When ActiveRecord, MySQL, and Your Data Betray You

This project is an example application to demonstrate the issues I will be discussing in one-on-one pairing sessions at the Big Ruby Conference held on Feb 20th - Feb 21, 2014 in Dallas, TX.

The application goes through each CRUD function

  • Create
  • Read
  • Update
  • Delete

and gives an example for an "oh_crud" problem (which is a dangerous or inefficient query) and a better optimized query. Often we assume that ActiveRecord knows best. If we really look at our queries we may discover that returned MySQL is not what we intended at all.

Generally small amounts of data, 0-3000 records won't demonstrate the problems well.I recommend using 10,000 or more records to really see the problems with these queries. Some you may not want to actually run because the server or MySQL may crash, but I'll go into that specifically in each function.

We will go over each CRUD function and explore the consequences of chaining, uses for Arel (A Relational Albegra) and even write raw SQL statements. Some of the examples will be based soley on producing faster queries, others will expore some assumptions we might make while writing out ActiveRecord queries.

Let's Make Some Data

This application uses Ruby 1.9.3 and Rails 4.0.2. Of course bundle install and rake db:migrate must be run before this will work.

lib/sample_data/ contains all the scripts to create the data to be used for each demonstrating function and an example for each CRUD function.

To create the CSV that will be used in the create method run the following command from your rails console.

SampleDataSetup.create_contacts_csv(10000, "contacts.csv", 1)

The first parameter is the amount, the second is the filename, and the third is which user to associate the contacts with.

The CSV function uses the Faker gem to create the data for the contacts.


To create the contacts in the database first run the above CSV creation command, it won't work without a file named contacts.csv.

The module demonstrates two creation methods. One that is uses the standard create! while the other demonstrates MySQL's Batch Insert - a function that is not available in ActiveRecord.

Let's run the oh_crud version.


This method runs through each row of the csv and creates the record:

def self.create_contacts_oh_crud
  CSV.foreach("#{Rails.root}/lib/sample_data/contacts.csv", headers: true) do |csv|
      :first_name => csv[0],
      :last_name => csv[1],
      :birthday => csv[2],
      :user_id => csv[24]

Now this method likely won't blow up with 10k records, but when you get closer to 100k records your time starts to suffer and you could be sitting there for hours waiting for it to finish.

This method produces the following SQL and benchmarks at:

(0.2ms)  BEGIN
SQL (0.6ms)  INSERT INTO `contacts` (`address_1`, `address_2`, `birthday`, `city`, `company`, `company_address_1`, `company_address_2`, `company_city`, `company_country`, `company_postal_code`, `company_state`, `country`, `email`, `facebook_account_link`, `first_name`, `github_account_link`, `gplus_account_link`, `last_name`, `linkedin_account_link`, `phone`, `postal_code`, `state`, `title`, `twitter_account_link`, `user_id`) VALUES ('6186 Charles Viaduct', 'Apt. 583', '1987-02-01', 'North Danikachester', 'Hickle LLC', '8904 Titus Squares', 'Apt. 371', 'West Otto', 'USA', '60254-1949', 'ID', 'USA', '', '', 'Lauretta', '', '', 'Senger', '', '1-439-658-7097 x274', '22882', 'AL', 'District Mobility Technician', '', 1)

         user     system      total         real
=>  44.740000   1.180000  45.920000 ( 51.095556)

I didn't post all of the SQL, imagine it does that for all 10k records, one at a time. Oof.

Now's let's delete that data and run the other create command.


And if we measure the other create method as we run it:

         user     system      total         real
=>   2.710000   0.050000   2.760000 (  3.227031)

WOW! That's a marked improvement. Now let's go over some caveats of using the MySQL Batch Insert. The batch insert works by inserting all the records at once.

NOTE: MySQL Batch Insert will not fire callbacks since each object is not instantiated and saved.

Instead of creating and saving each record the batch insert does exactly what it's name implies. There is no method in ActiveRecord that does exactly this. Don't be scared to abandon AR in the case, sometimes it's necessary and you're database will love you.

To get the batch insert working we'll start with an empty array. As we read each line of the csv we will build an array of contact values. We have to be very careful here to get the correct columns and that quotes are being escaped correctly. For my project I only cared that there weren't single quotes so I gsub'ed them out. You obviously can't do this with real data, this is just a demonstration.

The array must be in quotes and surrounded by parentheses ex "('data', 'data')". We then decide on a batch size - this is because even MySQL can't handle 10k records at once. After playing with a lot of different numbers I found that 2k records was most reliable. We then shift those values in the while loop until it's empty.

In the while loop we create the sql insert statement and join contacts_shifted for the values. Connect to mysql and excute.

This seems really complicated and it's a bit tedious. You have to be sure the values line up perfectly with the MySQL columns, but the savings are tremendous!

If you have a lot of data that needs to be created at once in MySQL, batch insert will likely be your best bet.

Before we can continue we need to set up the relationships between contacts and categories. These will be used later in the exercise and can be found in the SampleDataSetup module. The way these relationships are created is for ease of explanation and not the way it would be done in a live application. Simply run:



Now that we have our data, let's read it. Let's say we want to get all of our contacts first names. Generally we would run a simple each block on all contacts:


which looks like this:

def self.read_contacts_oh_crud
  Contact.where(:user_id => 1, :country => "USA").each do |contact|
    puts contact.first_name

And it benchmarks at:

         user     system      total         real
=>   0.910000   0.050000   0.960000 (  0.997945)

Simple enough. Not slow at all, but let's see if we can't speed this up. Let's try using find_each instead of each. find_each will collect the records in batches of 1000. What's interesting though without the where if we were to just ask for Contact.all.each turned out in Rails 4 to actually be a little faster than Contact.all.find_each. When using where though MySQL must search the records instead of just grabbing them all at once. Also as we increase records from say 10k to 100k the results will be very different.

So let's run the next method:


which looks like:

def self.read_contacts_optimized
  Contact.where(:user_id => 1, :country => "USA").find_each do |contact|
    puts contact.first_name

This one actually benchmarks a tad higher the first time it's run:

        user     system      total         real
=>   0.910000   0.050000   0.960000 (  0.990555)

It's hard to see the savings here, it's almost negilble. Interestingly, as the query becomes more complicated the savings are more and more noticble. I actually found (contrary to the Rails docs) that Contact.all.each ran faster than Contact.all.find_each.

So, let's look at the data we want. We're actually only outputting only one column so why collect the entire record? There's an even faster way to the entire record? Although this doesn't happen often we can grab just one column with the select Arel clause:


which looks like:

def self.read_contacts_optimized_alt
  Contact.where(:user_id => 1, :country => "USA").select(:first_name).each do |contact|
    puts contact.first_name

and benchmarks at:

         user     system      total         real
=>   0.560000   0.040000   0.600000 (  0.620693)

This one makes it easier to actually see the improvement. With 100k records this will save your database a LOT of time.


Oh no! We set all of our contacts to the wrong category! We need a quick way to update all of them. We could go through each category and update them. The following method will do this for us:

category = Category.first

which becomes:

def self.update_contacts_oh_crud(category)
  Categorization.all.each do |categorization|
    categorization.update_attributes(:category_id =>

But this is going to take a long time. It benchmarks at:

         user     system      total         real
=>  13.170000   1.280000  14.450000 ( 18.640788)

This takes so long because ActiveRecord instantiates and updates each individual object before it updates it. But all the records are getting the same update so maybe there is a better way that will update them all at once.

If we run this command:

category = Category.where(:name => "Coworkers").first

which is doing the following:

def self.update_contacts_optimized(category)
  Categorization.update_all(:category_id =>

and benchmarks it at:

         user     system      total         real
=>   0.000000   0.000000   0.000000 (  0.042140)

Instead of running through each object and updating them it produces a one line udpate in SQL:

UPDATE `categorizations` SET `categorizations`.`category_id` = 15

which updates all the records at once.

This optimized method provides huge time savings for your queries.


Now it's time to destroy records. Many developers would just run destroy_all on the model and call it a day. But what if you want to destroy related records? Associations with 10k records won't break MySQL but it will run slowly. With 100k records though, I've killed MySQL. That's not great. We never want our queries to be so overloading to the database that it just goes away. The problem with destroy_all is like our update methods above instantiates each object.

For these examples I recommend running rails console in sandbox mode (rails c -s) because then we can easily rollback our changes by exiting instead of re-running the create command.

A destroy all might look like this:


which will run:


This will instantiate and delete each individual categorization record. And it's benchmark demonstrates exactly how slow this is:

         user     system      total         real
=> 132.980000   0.370000 133.350000 (133.996548)

Now we could just change the query to:


That produces interesing SQL that I didn't expect the first time I ran the same query:

Contact Load (54.1ms)  SELECT `contacts`.* FROM `contacts` INNER JOIN `categorizations` ON `contacts`.`id` = `categorizations`.`contact_id` WHERE `categorizations`.`category_id` = 3
SQL (54.3ms)  DELETE FROM `categorizations` WHERE `categorizations`.`category_id` = 3 AND `categorizations`.`contact_id` IN (1, 2, 3, 4, 5, 6, 7, 8,...10000)

And benchmarks at:

         user     system      total         real
=> 129.550000   0.100000 129.650000 (129.727065)

This is better because we aren't instantiating each object, but it's confusing to other developers what we're trying to delete here.

Updating this to delete the categorization instead is a lot less confusing, so we can run:


It's faster but this isn't exactly what we want. If we look athe the SQL that is produced we're going to wonder if there's an even faster way to run this query. The first time I ran this I expected the following SQL to be produced:

DELETE FROM `categorizations` WHERE `categorizations`.`category_id` = 10

Now this is what we get if the relationship between category and categorizations is not actually loaded. But since that's rarely the case here's what we actually get:

DELETE FROM `categorizations` WHERE `categorizations`.`category_id` = 10 AND `categorizations`.`id` IN (10001, 10002, 10003, 10004,...

And benchmarks at:

         user     system      total         real
=> 130.080000   0.120000 130.200000 (130.308334)


Definitely not what I would have expected. After dealing with these types of issues at lot in the PhishMe application I decided it's better not to chain relationships when deleting records.

The following example will show that deleting the object directly is faster, safer, and actually produces the above expected SQL output:

If we run:


which does:

Categorization.where(:category_id =>

and produces the following MySQL and benchmark:

SQL (38.0ms)  DELETE FROM `categorizations` WHERE `categorizations`.`category_id` = 3
         user     system      total         real
=>   0.010000   0.000000   0.010000 (  0.043482)

The above is very clear what exactly we want from ActiveRecord and that we expect all categorizations with the category id to be deleted.

Now what if we really did want to delete all contacts that belonged to a specific category? Well there are lots of ways to do this but a simple join will be relatively quick and also be super clear about our intentions like the above query.

If we run:


which performs:

Contact.joins(:categorizations).where('categorizations.category_id' =>

and is translated into MySQL and benchmarks at:

SQL (2114.8ms)  DELETE FROM `contacts` WHERE `contacts`.`id` IN (SELECT id FROM (SELECT `contacts`.`id` FROM `contacts` INNER JOIN `categorizations` ON `categorizations`.`contact_id` = `contacts`.`id` WHERE `categorizations`.`category_id` = 3) __active_record_temp)
         user     system      total         real
=>   0.050000   0.010000   0.060000 (  2.170937)

All of these suggestions depend on your applications needs. It's important to keep in mind that with delete_all will not fire callbacks so dependent associations will not be deleted as expected.

It's important to keep in mind that we may be making assumptions about our query output when using ActiveRecord. It's an amazing tool but there is a lot of magic behind the scenes that can be detrmintal to your application if not used properly.


Project demonstrating issues with ActiveRecord, MySQL, and lots of data






No releases published


No packages published