Use strong_migrations gem

**Why**: `strong_migrations` is better maintained than
`zero_downtime_migrations` and seems to have better advice. The author
of the gem has written many other fantastic gems that are used in
production at Instacart, so they should be trustworthy.

The gem also comes with a handy rake task that will automatically
alphabetize the columns in `schema.rb`. This will eliminate issues
where after pulling the latest code and running recent migrations
locally, your schema.rb will change.

Gemfile

@@ -58,12 +58,12 @@ gem "sentry-raven"
 gem "shoryuken", "3.1.11"
 gem "sqlite3", platforms: [:ruby, :mswin, :mingw, :mswin, :x64_mingw]
 gem "stringex", require: false
+# catch problematic migrations at development/test time
+gem "strong_migrations"
 # execjs runtime
 gem "therubyracer", platforms: :ruby
 # Use Uglifier as compressor for JavaScript assets
 gem "uglifier", ">= 1.3.0"
-# catch problematic migrations at development/test time
-gem "zero_downtime_migrations"
 
 group :production, :staging, :ssh_forwarding, :development, :test do
   # Oracle DB

Gemfile.lock

@@ -464,6 +464,8 @@ GEM
       sprockets (>= 3.0.0)
     sqlite3 (1.3.13)
     stringex (2.8.5)
+    strong_migrations (0.3.1)
+      activerecord (>= 3.2.0)
     terminal-table (1.8.0)
       unicode-display_width (~> 1.1, >= 1.1.1)
     therubyracer (0.12.3)
@@ -496,8 +498,6 @@ GEM
       nokogiri (~> 1.5)
     xpath (3.0.0)
       nokogiri (~> 1.8)
-    zero_downtime_migrations (0.0.7)
-      activerecord
 
 PLATFORMS
   ruby
@@ -567,11 +567,11 @@ DEPENDENCIES
   sniffybara!
   sqlite3
   stringex
+  strong_migrations
   therubyracer
   timecop
   tzinfo-data
   uglifier (>= 1.3.0)
-  zero_downtime_migrations
 
 BUNDLED WITH
    1.17.2

Rakefile

@@ -7,3 +7,5 @@ require File.expand_path("config/application", __dir__)
 Dir[Rails.root.join("lib/tasks/support/**/*.rb")].each { |f| require f }
 
 Rails.application.load_tasks
+
+task "db:schema:dump": "strong_migrations:alphabetize_columns"

config/initializers/strong_migrations.rb

@@ -0,0 +1 @@
+StrongMigrations.start_after = 20190111000717

db/migrate/20190110220936_make_hearing_views_polymorphic.rb

@@ -1,13 +1,13 @@
 class MakeHearingViewsPolymorphic < ActiveRecord::Migration[5.1]
-  safety_assured
-
   def change
-    add_column :hearing_views, :hearing_type, :string
-    remove_index :hearing_views, [:hearing_id, :user_id]
-    add_index :hearing_views, [:hearing_id, :user_id, :hearing_type], unique: true
+    safety_assured do
+      add_column :hearing_views, :hearing_type, :string
+      remove_index :hearing_views, [:hearing_id, :user_id]
+      add_index :hearing_views, [:hearing_id, :user_id, :hearing_type], unique: true
 
-    HearingView.find_each do |hearing_view|
-      hearing_view.update!(hearing_type: "LegacyHearing")
+      HearingView.find_each do |hearing_view|
+        hearing_view.update!(hearing_type: "LegacyHearing")
+      end
     end
   end
 end