Add version constraint to database gem in generated application

We are using the same version constraint in the database adapters so
when a new version of the adapter that doesn't work with the version of
rails is released we don't break new applications.

railties/lib/rails/generators/app_base.rb

@@ -162,7 +162,8 @@ module Rails
 
       def database_gemfile_entry
         return [] if options[:skip_active_record]
-        GemfileEntry.version gem_for_database, nil,
+        gem_name, gem_version = gem_for_database
+        GemfileEntry.version gem_name, gem_version,
                             "Use #{options[:database]} as the database for Active Record"
       end
 
@@ -202,6 +203,16 @@ module Rails
         def self.path(name, path, comment = nil)
           new(name, nil, comment, path: path)
         end
+
+        def version
+          version = super
+
+          if version.is_a?(Array)
+            version.join("', '")
+          else
+            version
+          end
+        end
       end
 
       def rails_gemfile_entry
@@ -230,16 +241,16 @@ module Rails
       def gem_for_database
         # %w( mysql oracle postgresql sqlite3 frontbase ibm_db sqlserver jdbcmysql jdbcsqlite3 jdbcpostgresql )
         case options[:database]
-        when "oracle"         then "ruby-oci8"
-        when "postgresql"     then "pg"
-        when "frontbase"      then "ruby-frontbase"
-        when "mysql"          then "mysql2"
-        when "sqlserver"      then "activerecord-sqlserver-adapter"
-        when "jdbcmysql"      then "activerecord-jdbcmysql-adapter"
-        when "jdbcsqlite3"    then "activerecord-jdbcsqlite3-adapter"
-        when "jdbcpostgresql" then "activerecord-jdbcpostgresql-adapter"
-        when "jdbc"           then "activerecord-jdbc-adapter"
-        else options[:database]
+        when "oracle"         then ["ruby-oci8", nil]
+        when "postgresql"     then ["pg", ["~> 0.18"]]
+        when "frontbase"      then ["ruby-frontbase", nil]
+        when "mysql"          then ["mysql2", [">= 0.3.18", "< 0.5"]]
+        when "sqlserver"      then ["activerecord-sqlserver-adapter", nil]
+        when "jdbcmysql"      then ["activerecord-jdbcmysql-adapter", nil]
+        when "jdbcsqlite3"    then ["activerecord-jdbcsqlite3-adapter", nil]
+        when "jdbcpostgresql" then ["activerecord-jdbcpostgresql-adapter", nil]
+        when "jdbc"           then ["activerecord-jdbc-adapter", nil]
+        else [options[:database], nil]
         end
       end
 

railties/lib/rails/generators/rails/plugin/templates/%name%.gemspec

@@ -22,6 +22,6 @@ Gem::Specification.new do |s|
   <%= '# ' if options.dev? || options.edge? -%>s.add_dependency "rails", "~> <%= Rails::VERSION::STRING %>"
 <% unless  options[:skip_active_record] -%>
 
-  s.add_development_dependency "<%= gem_for_database %>"
+  s.add_development_dependency "<%= gem_for_database[0] %>"
 <% end  -%>
 end

railties/lib/rails/generators/rails/plugin/templates/Gemfile

@@ -11,7 +11,7 @@ gemspec
 
 <% if  options[:skip_gemspec] -%>
 group :development do
-  gem '<%= gem_for_database %>'
+  gem '<%= gem_for_database[0] %>'
 end
 <% else  -%>
 # Declare any dependencies that are still in development here instead of in

railties/test/generators/app_generator_test.rb

@@ -264,7 +264,7 @@ class AppGeneratorTest < Rails::Generators::TestCase
     if defined?(JRUBY_VERSION)
       assert_gem "activerecord-jdbcmysql-adapter"
     else
-      assert_gem "mysql2"
+      assert_gem "mysql2", "'>= 0.3.18', '< 0.5'"
     end
   end
 
@@ -279,7 +279,7 @@ class AppGeneratorTest < Rails::Generators::TestCase
     if defined?(JRUBY_VERSION)
       assert_gem "activerecord-jdbcpostgresql-adapter"
     else
-      assert_gem "pg"
+      assert_gem "pg", "'~> 0.18'"
     end
   end
 
@@ -686,7 +686,11 @@ class AppGeneratorTest < Rails::Generators::TestCase
     capture(:stdout) { generator.send(*args, &block) }
   end
 
-  def assert_gem(gem)
-    assert_file "Gemfile", /^\s*gem\s+["']#{gem}["']$*/
+  def assert_gem(gem, constraint = nil)
+    if constraint
+      assert_file "Gemfile", /^\s*gem\s+["']#{gem}["'], #{constraint}$*/
+    else
+      assert_file "Gemfile", /^\s*gem\s+["']#{gem}["']$*/
+    end
   end
 end