Each for each metadata field, there's now an automatically maintained "$field-lastchanged" that gives the timestamp of the last change to that field.

Note that this is a nearly entirely free feature. The data was already
stored in the metadata log in an easily accessible way, and already was
parsed to a time when parsing the log. The generation of the metadata
fields may even be done lazily, although probably not entirely (the map
has to be evaulated to when queried).

Annex/MetaData.hs

@@ -5,11 +5,15 @@
  - Licensed under the GNU GPL version 3 or higher.
  -}
 
-module Annex.MetaData where
+module Annex.MetaData (
+	genMetaData,
+	module X
+) where
 
 import Common.Annex
 import qualified Annex
-import Types.MetaData
+import Types.MetaData as X
+import Annex.MetaData.StandardFields as X
 import Logs.MetaData
 import Annex.CatFile
 
@@ -19,15 +23,6 @@ import Data.Time.Calendar
 import Data.Time.Clock
 import Data.Time.Clock.POSIX
 
-tagMetaField :: MetaField
-tagMetaField = mkMetaFieldUnchecked "tag"
-
-yearMetaField :: MetaField
-yearMetaField = mkMetaFieldUnchecked "year"
-
-monthMetaField :: MetaField
-monthMetaField = mkMetaFieldUnchecked "month"
-
 {- Adds metadata for a file that has just been ingested into the
  - annex, but has not yet been committed to git.
  -

Annex/MetaData/StandardFields.hs

@@ -0,0 +1,38 @@
+{- git-annex metadata, standard fields
+ -
+ - Copyright 2014 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Annex.MetaData.StandardFields (
+	tagMetaField,
+	yearMetaField,
+	monthMetaField,
+	lastChangedField,
+	isLastChangedField
+) where
+
+import Types.MetaData
+
+import Data.List
+
+tagMetaField :: MetaField
+tagMetaField = mkMetaFieldUnchecked "tag"
+
+yearMetaField :: MetaField
+yearMetaField = mkMetaFieldUnchecked "year"
+
+monthMetaField :: MetaField
+monthMetaField = mkMetaFieldUnchecked "month"
+
+lastChangedField :: MetaField -> MetaField
+lastChangedField f = mkMetaFieldUnchecked (fromMetaField f ++ lastchanged)
+
+isLastChangedField :: MetaField -> Bool
+isLastChangedField f = lastchanged `isSuffixOf` s && s /= lastchanged
+  where
+	s = fromMetaField f
+
+lastchanged :: String
+lastchanged = "-lastchanged"

Command/MetaData.hs

@@ -12,7 +12,6 @@ import qualified Annex
 import Command
 import Annex.MetaData
 import Logs.MetaData
-import Types.MetaData
 
 import qualified Data.Set as S
 import Data.Time.Clock.POSIX
@@ -84,7 +83,7 @@ perform :: POSIXTime -> [ModMeta] -> Key -> CommandPerform
 perform _ [] k = next $ cleanup k
 perform now ms k = do
 	oldm <- getCurrentMetaData k
-	let m = foldl' unionMetaData emptyMetaData $ map (modMeta oldm) ms
+	let m = combineMetaData $ map (modMeta oldm) ms
 	addMetaData' k m now
 	next $ cleanup k
 	

Logs/MetaData.hs

@@ -36,26 +36,50 @@ module Logs.MetaData (
 
 import Common.Annex
 import Types.MetaData
+import Annex.MetaData.StandardFields
 import qualified Annex.Branch
 import Logs
 import Logs.SingleValue
 
 import qualified Data.Set as S
+import qualified Data.Map as M
 import Data.Time.Clock.POSIX
+import Data.Time.Format
+import System.Locale
 
 instance SingleValueSerializable MetaData where
 	serialize = Types.MetaData.serialize
 	deserialize = Types.MetaData.deserialize
 
-getMetaData :: Key -> Annex (Log MetaData)
+getMetaDataLog :: Key -> Annex (Log MetaData)
-getMetaData = readLog . metaDataLogFile
+getMetaDataLog = readLog . metaDataLogFile
 
 {- Go through the log from oldest to newest, and combine it all
- - into a single MetaData representing the current state. -}
+ - into a single MetaData representing the current state.
+ -
+ - Automatically generates a lastchanged metadata for each field that's
+ - currently set, based on timestamps in the log.
+ -}
 getCurrentMetaData :: Key -> Annex MetaData
-getCurrentMetaData = currentMetaData . collect <$$> getMetaData
+getCurrentMetaData k = do
+	ls <- S.toAscList <$> getMetaDataLog k
+	let loggedmeta = currentMetaData $ combineMetaData $ map value ls
+	return $ currentMetaData $ unionMetaData loggedmeta
+		(lastchanged ls loggedmeta)
   where
-	collect = foldl' unionMetaData emptyMetaData . map value . S.toAscList
+	lastchanged ls (MetaData wanted) =
+		let m = foldl' (flip M.union) M.empty (map genlastchanged ls)
+		in MetaData $ M.mapKeys lastChangedField $
+			-- Only include fields that are currently set.
+			m `M.intersection` wanted
+	-- Makes each field have the timestamp as its value.
+	genlastchanged l =
+		let MetaData m = value l
+		    ts = S.singleton $ toMetaValue $ 
+		    	formatTime defaultTimeLocale "%s" $
+				posixSecondsToUTCTime $
+					changed l
+		in M.map (const ts) m
 
 {- Adds in some metadata, which can override existing values, or unset
  - them, but otherwise leaves any existing metadata as-is. -}
@@ -67,10 +91,12 @@ addMetaData k metadata = addMetaData' k metadata =<< liftIO getPOSIXTime
  - will tend to be generated across the different log files, and so
  - git will be able to pack the data more efficiently. -}
 addMetaData' :: Key -> MetaData -> POSIXTime -> Annex ()
-addMetaData' k metadata now = Annex.Branch.change (metaDataLogFile k) $
+addMetaData' k (MetaData m) now = Annex.Branch.change (metaDataLogFile k) $
 	showLog . simplifyLog 
 		. S.insert (LogEntry now metadata)
 		. parseLog
+  where
+	metadata = MetaData $ M.filterWithKey (\f _ -> not (isLastChangedField f)) m
 
 {- Simplify a log, removing historical values that are no longer
  - needed. 
@@ -148,7 +174,7 @@ copyMetaData :: Key -> Key -> Annex ()
 copyMetaData oldkey newkey
 	| oldkey == newkey = noop
 	| otherwise = do
-		l <- getMetaData oldkey
+		l <- getMetaDataLog oldkey
 		unless (S.null l) $
 			Annex.Branch.change (metaDataLogFile newkey) $
 				const $ showLog l

Types/MetaData.hs

@@ -28,6 +28,7 @@ module Types.MetaData (
 	emptyMetaData,
 	updateMetaData,
 	unionMetaData,
+	combineMetaData,
 	differenceMetaData,
 	isSet,
 	currentMetaData,
@@ -188,6 +189,9 @@ unionMetaData :: MetaData -> MetaData -> MetaData
 unionMetaData (MetaData old) (MetaData new) = MetaData $
 	M.unionWith S.union new old
 
+combineMetaData :: [MetaData] -> MetaData
+combineMetaData = foldl' unionMetaData emptyMetaData
+
 differenceMetaData :: MetaData -> MetaData -> MetaData
 differenceMetaData (MetaData m) (MetaData excludem) = MetaData $
 	M.differenceWith diff m excludem

debian/changelog

@@ -27,6 +27,9 @@ git-annex (5.20140307) UNRELEASED; urgency=medium
   * map: Fix crash when one of the remotes of a repo is a local directory
     that does not exist, or is not a git repo.
   * rsync special remote: Fix slashes when used on Windows.
+  * Each for each metadata field, there's now an automatically maintained
+    "$field-lastchanged" that gives the timestamp of the last change to that
+    field.
 
  -- Joey Hess <joeyh@debian.org>  Thu, 06 Mar 2014 16:17:01 -0400
 

doc/metadata.mdwn

@@ -23,11 +23,14 @@ The field names are limited to alphanumerics (and `[_-.]`), and are case
 insensitive. The metadata values can contain absolutely anything you
 like -- but you're recommended to keep it simple and reasonably short.
 
-Here are some recommended metadata fields to use:
+Here are some metadata fields that git-annex has special support for:
 
 * `tag` - With each tag being a different value.
 * `year`, `month` - When this particular version of the file came into
   being.
+* `$field-lastchanged` - This is automatically maintained for each
+  field that's set, and gives the time stamp (since the Unix epoch)
+  of the most recent change to the field. It cannot be modified directly.
 
 To make git-annex automatically set the year and month when adding files,
 run `git config annex.genmetadata true`. Also, see

doc/todo/wishlist:_metadata_metadata_view.mdwn

@@ -18,3 +18,6 @@ Something along the lines of
 This would allow me to review files that haven't had any tag changes applied for a while and thus, may need the tags updating.
 
 I've done this in every tagging system I've used by (ab)using mtime, but that requires an additional step (of touching the file).
+
+> [[done]]; "$field-lastchanged" is automatically made available for each
+> field! --[[Joey]]