Add an option database_type

Problem: When using `numeric_ordering: true`, closure_tree tries to find out the type of database during the initialization process. This can be problematic, for example when doing `rails db:create`, in which case there is no database, yet. Another scenario is `rails asssets:prcompile` in a Docker container that is being made for production, but it doesn't have access to the production database yet, because it isn't rolled out, yet.

Solution: I have added an option to specify the database_type manually. This option is *only* used then numeric_ordering is set to `true`

lib/closure_tree/has_closure_tree.rb

@@ -11,7 +11,8 @@ def has_closure_tree(options = {})
         :dont_order_roots,
         :numeric_order,
         :touch,
-        :with_advisory_lock
+        :with_advisory_lock,
+        :database_type
       )
 
       class_attribute :_ct

lib/closure_tree/numeric_order_support.rb

@@ -12,6 +12,17 @@ def self.adapter_for_connection(connection)
       end
     end
 
+    def self.adapter_for_database_type(database_type)
+      case database_type
+      when :postgres
+        ::ClosureTree::NumericOrderSupport::PostgreSQLAdapter
+      when :mysql
+        ::ClosureTree::NumericOrderSupport::MysqlAdapter
+      else
+        ::ClosureTree::NumericOrderSupport::GenericAdapter
+      end
+    end
+
     module MysqlAdapter
       def reorder_with_parent_id(parent_id, minimum_sort_order_value = nil)
         return if parent_id.nil? && dont_order_roots

lib/closure_tree/support.rb

@@ -27,7 +27,8 @@ def initialize(model_class, options)
       }.merge(options)
       raise ArgumentError, "name_column can't be 'path'" if options[:name_column] == 'path'
       if order_is_numeric?
-        extend NumericOrderSupport.adapter_for_connection(connection)
+        database_type = database_type_from_options || database_type_from_connection
+        extend NumericOrderSupport.adapter_for_database_type(database_type)
       end
     end
 
@@ -169,5 +170,16 @@ def create(model_class, attributes)
     def create!(model_class, attributes)
       create(model_class, attributes).tap { |ea| ea.save! }
     end
+
+    def database_type_from_connection
+      das = WithAdvisoryLock::DatabaseAdapterSupport.new(connection)
+      if das.postgresql?
+        :postgres
+      elsif das.mysql?
+        :mysql
+      else
+        :generic
+      end
+    end
   end
 end

lib/closure_tree/support_flags.rb

@@ -31,5 +31,9 @@ def has_inheritance_column?(hash = columns_hash)
     def has_name?
       model_class.new.attributes.include? options[:name_column]
     end
+
+    def database_type_from_options
+      options[:database_type]
+    end
   end
 end