After a fun bit of traipsing all over the Rails codebase, I found a few things out about bot Postgresql, Rails’ schema format and Rails’ convention around primary keys that I thought would be good to document for future me.

Rails has a schema format that looks similar to its migration format. Example:

create_table "users", force: true do |t|
  t.citext   "email"
  t.datetime "created_at"
  t.datetime "updated_at"
  # ... etc ...

This is pretty cool unless you’re doing something different from the standard conventions, like naming the primary key something different and changing its type. For example, Postgres has the uuid field type. This is a pretty standard key format convention for global unique identifiers (GUIDs), and, hey, let’s use them!

Rails provides for a means of doing this out-of-the-box with Postgres:

create_table "users", id: :uuid force: true do |t|
  # ... etc ...

Great! What does that do? Inspecting the database reveals two things:

  1. It creates the column id of type uuid and makes it a PRIMARY KEY
  2. It sets the default to be auto-generated via the uuid_generate_v4() method, which is part of the uuid-ossp extention in Postgres. This allows PKs to be generated on the fly uniquely, similarly to the auto-incrementing sequence field type.

So that’s cool, and works generally for most people’s use-case. But let’s say for example we want to change the PK’s name. Something like my_cool_pk. Rails provides at least three ways of doing this via migrations:

create_table "users", id: false, primary_key: "my_cool_pk"

# Or...

create_table "users", id: false do |t|
  t.primary_key :my_cool_pk, :uuid
  # ...

# Or...

create_table "users", id: false do |t|
  t.uuid :my_cool_pk, primary_key: true
  # ...

What you may notice from the first is that the PK column type gets dropped from the definition. Super important detail. The other two work just fine though. HOWEVER, both of the second two mechanisms, while they create the table just fine when running migrations, the schema.rb that all three methods generate all end up looking exactly like the first statement:

create_table "users", id: false, primary_key: "my_cool_pk"

Which means, if you load this schema into the database, Rails will use the default PK convention of setting the column type as sequence. Why is this a problem?

The first problem is repeatability. The only way to create the same DB you have in development is to run all of the migrations. The schema.rb file has this big disclaimer at the top of the file:

Note that this schema.rb definition is the authoritative source for your database schema. If you need to create the application database on another system, you should be using db:schema:load, not running all the migrations from scratch. The latter is a flawed and unsustainable approach (the more migrations you’ll amass, the slower it’ll run and the greater likelihood for issues).

It’s strongly recommended that you check this file into your version control system.

The glaring problem with this statement is that, because of the way this schema is generated for our special case, the schema file no longer represents our database structure. So what’s a database to do?

There are two options, a good one, and a better one. The good one involves creating a table with no primary key, a uuid column, and a mix of constraints and an index that mimic the properties of PRIMARY KEY. Here’s what that looks like in Rails migration land”

create_table "users", id: false "my_cool_pk" do |t|
  t.uuid :my_cool_pk, null: false   # you can add the default of uuid_generate_v4() if you like

create_index :users, :my_cool_pk, unique: true

This basically makes my_cool_pk follow all the same conventions as a PRIMARY KEY field. You can see from the Postgres documentation that this is basically what PRIMARY KEY does (though some sources tell me that it is implemented with some differences). “But Luke”, you say, as a responsible developer should, “what about the performance?”. I’m glad you asked. I ran (albiet through Rails ActiveRecord) some quick benchmarks to see how selection over an index faired over a PRIMARY KEY field. The following is my test code and benchmark results.

user_ids = []
other_user_ids = []

  id = SecureRandom.uuid

  user_ids <<
  other_user_ids <<

Benchmark.ips do |x|
  x.time = 10'index') { User.find user_ids.sample }'pk') { OtherUser.find other_user_ids.sample }!

# Calculating -------------------------------------
#               index    20.000  i/100ms
#                  pk    20.000  i/100ms
# -------------------------------------------------
#               index    214.927  (± 7.0%) i/s -      2.140k
#                  pk    217.380  (± 9.2%) i/s -      2.160k
# Comparison:
#                  pk:      217.4 i/s
#               index:      214.9 i/s - 1.01x slower

So the resulting performance between the two field types (PRIMARY KEY vs index + constraints) have negligible performance impact. So that’s an option.

However, there’s one other option to get Rails to cooperate: Ditch the troublesome schema.rb. This option is helpful if you want to still want to follow good DB practice while still using Rails’ db sync mechanisms. The main one being test db preperation. Rails has/had/has again a db:test:prepare task which loads the test database from the schema if there are any migrations pending on the test db.

task :prepare => %w(environment load_config) do
  unless ActiveRecord::Base.configurations.blank?

And the test:load action:

task :load => %w(db:test:purge) do
  case ActiveRecord::Base.schema_format
    when :ruby
    when :sql

So Rails can use an actual SQL structure file instead of the schema file, so long as you’ve set config.active_record.schema_format = :sql in your application.rb config file. This, from all the investigation I’ve made so far, does the Right Thing™ in generating the correct PRIMARY KEY field with the correct uuid type.

    my_cool_pk uuid DEFAULT uuid_generate_v4() NOT NULL
    # ... etc ...

    ADD CONSTRAINT users_pkey PRIMARY KEY (my_cool_pk);

This means the db:test:load/prepare tasks will work normally. The only thing that breaks is you can no longer use db:schema:load. However, the db:structure:load task will work just as well.


Short Twitter convo with a few Heroku engineers on the core PG team: