DODB.no_index, Storage#new_nilable_index.

The APIs are still a bit experimental at this point, but the feature is
very likely to stay.

src/dodb.cr

@@ -76,6 +76,12 @@ class DODB::DataBase(V)
 		end
 	end
 
+	def new_nilable_index(name : String, &block : Proc(V, String | DODB::NoIndex))
+		Index(V).new(self, @directory_name, name, block).tap do |indexer|
+			@indexers << indexer
+		end
+	end
+
 	def new_tags(name : String, &block : Proc(V, Array(String)))
 		Tags(V).new(@directory_name, name, block).tap do |tags|
 			@indexers << tags

src/dodb/index.cr

@@ -6,7 +6,7 @@ require "./indexer.cr"
 
 class DODB::Index(V) < DODB::Indexer(V)
 	property name         : String
-	property key_proc     : Proc(V, String)
+	property key_proc     : Proc(V, String | NoIndex) | Proc(V, String)
 	getter   storage_root : String
 
 	@storage : DODB::DataBase(V)
@@ -34,6 +34,8 @@ class DODB::Index(V) < DODB::Indexer(V)
 	def index(key, value)
 		index_key = key_proc.call value
 
+		return if index_key.is_a? NoIndex
+
 		symlink = file_path_index index_key
 
 		Dir.mkdir_p ::File.dirname symlink
@@ -49,6 +51,8 @@ class DODB::Index(V) < DODB::Indexer(V)
 	def deindex(key, value)
 		index_key = key_proc.call value
 
+		return if index_key.is_a? NoIndex
+
 		symlink = file_path_index index_key
 
 		::File.delete symlink
@@ -105,6 +109,9 @@ class DODB::Index(V) < DODB::Indexer(V)
 	# in case new_value hasn't changed its index
 	def update(new_value : V)
 		index = key_proc.call new_value
+
+		raise Exception.new "new value is not indexable" if index.is_a? NoIndex
+
 		update index, new_value
 	end
 

src/dodb/no_index.cr

@@ -0,0 +1,8 @@
+
+class DODB::NoIndex
+end
+
+module DODB
+	class_getter no_index = NoIndex.new
+end
+