Add ability to override just the metadata in a struct.

service/datastore/properties.go

Index: service/datastore/properties.go
diff --git a/service/datastore/properties.go b/service/datastore/properties.go
index 60d8b423ad6dfc4fe7399a0570df4e7fbba883d8..9746c67b1d8e4d920023ef0528570bf79c821073 100644
--- a/service/datastore/properties.go
+++ b/service/datastore/properties.go
@@ -680,20 +680,36 @@ type PropertyLoadSaver interface {
 	// which was held by this PropertyLoadSaver.
 	Save(withMeta bool) (PropertyMap, error)
 
+	MetaGetterSetter
+
+	// Problem indicates that this PLS has a fatal problem. Usually this is
+	// set when the underlying struct has recursion, invalid field types, nested
+	// slices, etc.
+	Problem() error
+}
+
+// MetaGetterSetter is the subset of PropertyLoadSaver which pertains to
+// getting and saving metadata.
+//
+// A *struct may implement this interface to provide metadata which is
+// supplimental to the variety described by GetPLS. For example, this could be
+// used to implement a parsed-out $kind or $id.
+type MetaGetterSetter interface {
 	MetaGetter
 
 	// GetAllMeta returns a PropertyMap with all of the metadata in this
-	// PropertyLoadSaver. If a metadata field has an error during serialization,
+	// MetaGetterSetter. If a metadata field has an error during serialization,
 	// it is skipped.
+	//
+	// If a *struct is implementing this, then it only needs to return the
+	// metadata fields which would be returned by its GetMeta implementation, and
+	// the `GetPLS` implementation will add any statically-defined metadata
+	// fields. So if GetMeta provides $id, but there's a simple tagged field for
+	// $kind, this method is only expected to return a PropertyMap with "$id".
 	GetAllMeta() PropertyMap

[Vadim Sh., 2015/10/13 04:27:48]

hm, why this isn't in MetaGetter?

[iannucci, 2015/10/13 04:34:40]

It's... for not a super great reason. Basically when you to GetMulti, you have
the option to pass down a restricted subset of PLS... just the bits to Get
metadata (a MultiMetaGetter). To make this object as simple as possible,
GetAllMeta isn't included.

This could be revised at some point, but I'm inclined to leave it alone for now.
If someone needed GetAllMeta in e.g. a filter, then it could be moved into
MetaGetter, but at that point it's very likely an abstraction violation; filters
should be able to know precisely what metadata they need.

[Vadim Sh., 2015/10/13 04:39:59]

It just looks odd: *Setter (that extends Getter) adding another Get-only method.

 
 	// SetMeta allows you to set the current value of the meta-keyed field.
 	SetMeta(key string, val interface{}) error
-
-	// Problem indicates that this PLS has a fatal problem. Usually this is
-	// set when the underlying struct has recursion, invalid field types, nested
-	// slices, etc.
-	Problem() error
 }
 
 // PropertyMap represents the contents of a datastore entity in a generic way.