Fix Common implementation.

src/dodb/storage/common.cr

@@ -33,7 +33,7 @@
 #
 # NOTE: fast for frequently requested data and requires a stable (and configurable) amount of memory.
 class DODB::Storage::Common(V) < DODB::Storage::Cached(V)
-	# The *fifo* an `EfficientFIFO` instance where the key of the requested data is pushed.
+	# The *fifo* is an instance of `EfficientFIFO` where the key of the requested data is pushed.
 	# In case the number of stored entries exceeds what is allowed, the least recently used entry is removed.
 	property fifo : EfficientFIFO(Int32)
 
@@ -50,8 +50,15 @@ class DODB::Storage::Common(V) < DODB::Storage::Cached(V)
 		end
 	end
 
+	# Verifies that the value is in cache, or read it on disk.
+	# Pushes the key in the fifo.
 	def [](key : Int32) : V
-		val = @data[key] rescue raise MissingEntry.new(key)
+		val = @data[key]?
+		if val.nil?
+			raise MissingEntry.new(key) unless ::File.exists? file_path key
+			val = read file_path key
+			@data[key] = val
+		end
 		push_fifo key
 		val
 	end