Add date_trunc function to MySQL

.gitignore

@@ -6,5 +6,7 @@
 /doc/
 /pkg/
 /spec/reports/
+.ruby-version
+.byebug_history
 /tmp/
 .DS_STORE

README.md

@@ -219,7 +219,7 @@ class PhoneFactModel < ActiveReporting::FactModel
 end
 ```
 
-### Implicit hierarchies with datetime columns (PostgreSQL support only)
+### Implicit hierarchies with datetime columns (PostgreSQL and MySQL* support only)
 
 The fastest approach to group by certain date metrics is to create so-called "date dimensions". For
 those Postgres users that are restricted from organizing their data in this way, Postgres provides
@@ -235,7 +235,12 @@ end
 
 When creating a metric, ActiveReporting will recognize implicit hierarchies for this dimension. The hierarchies correspond to the [values](https://www.postgresql.org/docs/8.1/static/functions-datetime.html#FUNCTIONS-DATETIME-TRUNC) supported by PostgreSQL. (See example under the metric section, below.)
 
-*NOTE*: PRs welcomed to support this functionality in other databases.
+*Before Using with MySQL
+You can use `date_trunc` function to a rails app. We have a migration generator that will generate a SQL
+statement to create it on your database. To do it run this command:
+```
+bin/rails generate active_reporting:mysql_function_migration
+```
 
 ## Configuring Dimension Filters
 

lib/active_reporting/reporting_dimension.rb

@@ -4,7 +4,7 @@
 module ActiveReporting
   class ReportingDimension
     extend Forwardable
-    SUPPORTED_DBS = %w[PostgreSQL PostGIS].freeze
+    SUPPORTED_DBS = %w[PostgreSQL PostGIS Mysql2].freeze
     # Values for the Postgres `date_trunc` method.
     # See https://www.postgresql.org/docs/10/static/functions-datetime.html#FUNCTIONS-DATETIME-TRUNC
     DATETIME_HIERARCHIES = %i[microseconds milliseconds second minute hour day week month quarter year decade

lib/generators/active_reporting/mysql_function_migration_generator.rb

@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+require 'rails/generators/active_record'
+
+module ActiveReporting
+  class MysqlFunctionMigrationGenerator < ActiveRecord::Generators::Base
+    argument :name, type: :string, default: 'no_name', banner: 'This migration has default name'
+    source_root File.expand_path("../templates", __FILE__)
+
+    def copy_active_reporting_migration
+      if mysql? && !migration_exists?
+        migration_template(
+          'migration.rb',
+          "#{migration_path}/add_date_trunc_function_to_mysql.rb",
+          migration_version: migration_version
+        )
+      end
+    end
+
+    def migration_exists?
+      Dir.glob(
+        "#{File.join(destination_root, migration_path)}/[0-9]*_*.rb"
+      ).grep(/\d+_add_date_trunc_function_to_mysql.rb$/).present?
+    end
+
+    def rails5_and_up?
+      Rails::VERSION::MAJOR >= 5
+    end
+
+    def mysql?
+      config = ActiveRecord::Base.configurations[Rails.env]
+      config && config['adapter'] == 'mysql2'
+    end
+
+    def migration_version
+      if rails5_and_up?
+        "[#{Rails::VERSION::MAJOR}.#{Rails::VERSION::MINOR}]"
+      end
+    end
+
+    def migration_path
+      if Rails.version >= '5.0.3'
+        db_migrate_path
+      else
+        @migration_path ||= File.join('db', 'migrate')
+      end
+    end
+  end
+end

lib/generators/active_reporting/templates/migration.rb

@@ -0,0 +1,44 @@
+# frozen_string_literal: true
+
+class AddDateTruncFunctionToMysql < ActiveRecord::Migration[5.1]
+  def up
+    drop_function
+    ActiveRecord::Base.connection.execute(date_trunc_function.strip.gsub(/\n/, ' '))
+  end
+
+  def down
+    drop_function
+  end
+
+  private
+
+  def drop_function
+    ActiveRecord::Base.connection.execute(
+      <<-SQL
+        DROP FUNCTION IF EXISTS date_trunc
+      SQL
+    )
+  end
+
+  def date_trunc_function
+    func = <<-SQL
+        CREATE FUNCTION date_trunc(vInterval varchar(7), vDate timestamp)
+        RETURNS timestamp
+        begin
+          declare toReturn timestamp;
+
+          IF vInterval = 'year' then set toReturn = date_add('1900-01-01', interval TIMESTAMPDIFF(YEAR, '1900-01-01', vDate) YEAR);
+          elseif vInterval = 'quarter' then set toReturn = date_add('1900-01-01', interval TIMESTAMPDIFF(QUARTER, '1900-01-01', vDate) QUARTER);
+          elseif vInterval = 'month' then set toReturn = date_add('1900-01-01', interval TIMESTAMPDIFF(MONTH, '1900-01-01', vDate) MONTH);
+          elseif vInterval = 'week' then set toReturn = date_add('1900-01-01', interval TIMESTAMPDIFF(WEEK, '1900-01-01', vDate) WEEK);
+          elseif vInterval = 'day' then set toReturn = date_add('1900-01-01', interval TIMESTAMPDIFF(DAY, '1900-01-01', vDate) DAY);
+          elseif vInterval = 'hour' then set toReturn = date_add('1900-01-01', interval TIMESTAMPDIFF(HOUR, '1900-01-01', vDate) HOUR);
+          elseif vInterval = 'minute' then set toReturn = date_add('1900-01-01', interval TIMESTAMPDIFF(MINUTE, '1900-01-01', vDate) MINUTE);
+          END IF;
+
+          RETURN toReturn;
+        END
+      SQL
+    func
+  end
+end

test/active_reporting/report_test.rb

@@ -32,8 +32,10 @@ def test_report_runs_with_an_aggregate_other_than_count
     assert data.all? { |r| r.key?('a_metric') }
   end
 
+  # NOTE: In order to make this test pass with `mysql` you need to add
+  # `date_trunc` function to your local mysql database
   def test_report_runs_with_a_date_grouping
-    if ENV['DB'] == 'pg'
+    if ENV['DB'] == 'pg' || ENV['DB'] == 'mysql'
       metric = ActiveReporting::Metric.new(:a_metric, fact_model: UserFactModel, dimensions: [{created_at: :month}])
       report = ActiveReporting::Report.new(metric)
       data = report.run

test/active_reporting/reporting_dimension_test.rb

@@ -87,7 +87,7 @@ def test_label_can_be_passed_in_if_dimension_is_herarchical
   def test_label_can_be_passed_in_if_dimension_is_datetime
     refute @user_dimension.hierarchical?
     assert @user_dimension.type == ActiveReporting::Dimension::TYPES[:degenerate]
-    if ENV['DB'] == 'pg'
+    if ENV['DB'] == 'pg' || ENV['DB'] == 'mysql'
       ActiveReporting::ReportingDimension.new(@user_dimension, label: :year)
     else
       assert_raises ActiveReporting::InvalidDimensionLabel do

test/test_helper.rb

@@ -12,6 +12,7 @@
 require 'minitest/pride'
 
 db = ENV['DB'] || 'sqlite'
+db_user = ENV['DB_USER']
 case db
 when 'pg'
   ActiveRecord::Base.establish_connection(
@@ -25,7 +26,8 @@
   ActiveRecord::Base.establish_connection(
     adapter:  'mysql2',
     database: 'active_reporting_test',
-    encoding: 'utf8'
+    encoding: 'utf8',
+    username: db_user
   )
 when 'sqlite'
   ActiveRecord::Base.establish_connection(