fix: build queue and dependencies + add key_map_fn

lektor_groupby/backref.py

@@ -53,16 +53,6 @@ class VGroups:
         order_by: Union[str, Iterable[str], None] = None,
     ) -> Iterator['GroupBySource']:
         ''' Extract all referencing groupby virtual objects from a page. '''
-        ctx = get_ctx()
-        if not ctx:
-            raise NotImplementedError("Shouldn't happen, where is my context?")
-        # get GroupBy object
-        builder = ctx.build_state.builder
-        groupby = GroupByRef.of(builder)
-        groupby.make_once(builder)  # ensure did cluster before
-        # manage config dependencies
-        for dep in groupby.dependencies:
-            ctx.record_dependency(dep)
         # prepare filter
         if isinstance(keys, str):
             keys = [keys]
@@ -70,6 +60,13 @@ class VGroups:
             fields = [fields]
         if isinstance(flows, str):
             flows = [flows]
+        # get GroupBy object
+        ctx = get_ctx()
+        if not ctx:
+            raise NotImplementedError("Shouldn't happen, where is my context?")
+        builder = ctx.build_state.builder
+        # TODO: fix record_dependency -> process in non-capturing context
+        GroupByRef.of(builder).make_once(keys)  # ensure did cluster before use
         # find groups
         proc_list = [record]
         done_list = set()  # type: Set[GroupBySource]
@@ -86,6 +83,13 @@ class VGroups:
                     continue
                 done_list.add(vobj())
 
+        # manage config dependencies
+        deps = set()  # type: Set[str]
+        for vobj in done_list:
+            deps.update(vobj.config.dependencies)
+        for dep in deps:
+            ctx.record_dependency(dep)
+
         if order_by:
             if isinstance(order_by, str):
                 order = split_strip(order_by, ',')  # type: Iterable[str]

lektor_groupby/config.py

@@ -1,12 +1,33 @@
 from inifile import IniFile
-from lektor.utils import slugify
+from lektor.environment import Expression
+from lektor.context import Context
+from lektor.utils import slugify as _slugify
+from typing import TYPE_CHECKING
+from typing import Set, Dict, Optional, Union, Any, List, Generator
 from .util import split_strip
+if TYPE_CHECKING:
+    from lektor.sourceobj import SourceObject
 
-from typing import Set, Dict, Optional, Union, Any, List
 
 AnyConfig = Union['Config', IniFile, Dict]
 
 
+class ConfigError(Exception):
+    ''' Used to print a Lektor console error. '''
+
+    def __init__(
+        self, key: str, field: str, expr: str, error: Union[Exception, str]
+    ):
+        self.key = key
+        self.field = field
+        self.expr = expr
+        self.error = error
+
+    def __str__(self) -> str:
+        return 'Invalid config for [{}.{}] = "{}"  –  Error: {}'.format(
+            self.key, self.field, self.expr, repr(self.error))
+
+
 class Config:
     '''
     Holds information for GroupByWatcher and GroupBySource.
@@ -22,11 +43,15 @@ class Config:
         root: Optional[str] = None,  # default: "/"
         slug: Optional[str] = None,  # default: "{attr}/{group}/index.html"
         template: Optional[str] = None,  # default: "groupby-{attr}.html"
+        replace_none_key: Optional[str] = None,  # default: None
+        key_map_fn: Optional[str] = None,  # default: None
     ) -> None:
         self.key = key
         self.root = (root or '/').rstrip('/') or '/'
         self.slug = slug or (key + '/{key}/')  # key = GroupBySource.key
         self.template = template or f'groupby-{self.key}.html'
+        self.replace_none_key = replace_none_key
+        self.key_map_fn = key_map_fn
         # editable after init
         self.enabled = True
         self.dependencies = set()  # type: Set[str]
@@ -37,7 +62,8 @@ class Config:
 
     def slugify(self, k: str) -> str:
         ''' key_map replace and slugify. '''
-        return slugify(self.key_map.get(k, k))  # type: ignore[no-any-return]
+        rv = self.key_map.get(k, k)
+        return _slugify(rv) or rv  # the `or` allows for example "_"
 
     def set_fields(self, fields: Optional[Dict[str, Any]]) -> None:
         '''
@@ -72,7 +98,7 @@ class Config:
 
     def __repr__(self) -> str:
         txt = '<GroupByConfig'
-        for x in ['key', 'root', 'slug', 'template', 'enabled']:
+        for x in ['enabled', 'key', 'root', 'slug', 'template', 'key_map_fn']:
             txt += ' {}="{}"'.format(x, getattr(self, x))
         txt += f' fields="{", ".join(self.fields)}"'
         if self.order_by:
@@ -87,6 +113,8 @@ class Config:
             root=cfg.get('root'),
             slug=cfg.get('slug'),
             template=cfg.get('template'),
+            replace_none_key=cfg.get('replace_none_key'),
+            key_map_fn=cfg.get('key_map_fn'),
         )
 
     @staticmethod
@@ -116,3 +144,56 @@ class Config:
             return Config.from_ini(key, config)
         elif isinstance(config, Dict):
             return Config.from_dict(key, config)
+
+    # -----------------------------------
+    #          Field Expressions
+    # -----------------------------------
+
+    def _make_expression(self, expr: Any, *, on: 'SourceObject', field: str) \
+            -> Union[Expression, Any]:
+        ''' Create Expression and report any config error. '''
+        if not isinstance(expr, str):
+            return expr
+        try:
+            return Expression(on.pad.env, expr)
+        except Exception as e:
+            raise ConfigError(self.key, field, expr, e)
+
+    def eval_field(self, attr: str, *, on: 'SourceObject') \
+            -> Union[Expression, Any]:
+        ''' Create an expression for a custom defined user field. '''
+        # do not `gather_dependencies` because fields are evaluated on the fly
+        # dependency tracking happens whenever a field is accessed
+        return self._make_expression(
+            self.fields[attr], on=on, field='fields.' + attr)
+
+    def eval_slug(self, key: str, *, on: 'SourceObject') -> Optional[str]:
+        ''' Either perform a "{key}" substitution or evaluate expression. '''
+        cfg_slug = self.slug
+        if not cfg_slug:
+            return None
+        if '{key}' in cfg_slug:
+            if key:
+                return cfg_slug.replace('{key}', key)
+            else:
+                raise ConfigError(self.key, 'slug', cfg_slug,
+                                  'Cannot replace {key} with None')
+                return None
+        else:
+            # TODO: do we need `gather_dependencies` here too?
+            expr = self._make_expression(cfg_slug, on=on, field='slug')
+            return expr.evaluate(on.pad, this=on, alt=on.alt) or None
+
+    def eval_key_map_fn(self, *, on: 'SourceObject', context: Dict) -> Any:
+        '''
+        If `key_map_fn` is set, evaluate field expression.
+        Note: The function does not check whether `key_map_fn` is set.
+        Return: A Generator result is automatically unpacked into a list.
+        '''
+        exp = self._make_expression(self.key_map_fn, on=on, field='key_map_fn')
+        with Context(pad=on.pad) as ctx:
+            with ctx.gather_dependencies(self.dependencies.add):
+                res = exp.evaluate(on.pad, this=on, alt=on.alt, values=context)
+        if isinstance(res, Generator):
+            res = list(res)  # unpack for 1-to-n replacement
+        return res

lektor_groupby/groupby.py

@@ -1,6 +1,7 @@
 from lektor.builder import PathCache
 from lektor.db import Record  # isinstance
-from typing import TYPE_CHECKING, Set, List
+from lektor.reporter import reporter  # build
+from typing import TYPE_CHECKING, List, Optional, Iterable
 from .config import Config
 from .watcher import Watcher
 if TYPE_CHECKING:
@@ -19,14 +20,14 @@ class GroupBy:
     '''
 
     def __init__(self, resolver: 'Resolver') -> None:
+        self._building = False
         self._watcher = []  # type: List[Watcher]
         self._results = []  # type: List[GroupBySource]
         self.resolver = resolver
-        self.didBuild = False
 
     @property
-    def isNew(self) -> bool:
+    def isBuilding(self) -> bool:
-        return not self.didBuild
+        return self._building
 
     def add_watcher(self, key: str, config: 'AnyConfig') -> Watcher:
         ''' Init Config and add to watch list. '''
@@ -34,15 +35,8 @@ class GroupBy:
         self._watcher.append(w)
         return w
 
-    def get_dependencies(self) -> Set[str]:
-        deps = set()  # type: Set[str]
-        for w in self._watcher:
-            deps.update(w.config.dependencies)
-        return deps
-
     def queue_all(self, builder: 'Builder') -> None:
         ''' Iterate full site-tree and queue all children. '''
-        self.dependencies = self.get_dependencies()
         # remove disabled watchers
         self._watcher = [w for w in self._watcher if w.config.enabled]
         if not self._watcher:
@@ -61,16 +55,27 @@ class GroupBy:
             if isinstance(record, Record):
                 for w in self._watcher:
                     if w.should_process(record):
-                        w.process(record)
+                        w.remember(record)
 
-    def make_once(self, builder: 'Builder') -> None:
+    def make_once(self, filter_keys: Optional[Iterable[str]] = None) -> None:
-        ''' Perform groupby, iter over sources with watcher callback. '''
+        '''
-        self.didBuild = True
+        Perform groupby, iter over sources with watcher callback.
-        if self._watcher:
+        If `filter_keys` is set, ignore all other watchers.
+        '''
+        if not self._watcher:
+            return
+        # not really necessary but should improve performance of later reset()
+        if not filter_keys:
             self.resolver.reset()
+        remaining = []
         for w in self._watcher:
-                root = builder.pad.get(w.config.root)
+            # only process vobjs that are used somewhere
-                for vobj in w.iter_sources(root):
+            if filter_keys and w.config.key not in filter_keys:
+                remaining.append(w)
+                continue
+            self.resolver.reset(w.config.key)
+            # these are used in the current context (or on `build_all`)
+            for vobj in w.iter_sources():
                 # add original source
                 self._results.append(vobj)
                 self.resolver.add(vobj)
@@ -78,14 +83,30 @@ class GroupBy:
                 for sub_vobj in vobj.__iter_pagination_sources__():
                     self._results.append(sub_vobj)
                     self.resolver.add(sub_vobj)
-            self._watcher.clear()
+        # TODO: if this should ever run concurrently, pop() from watchers
+        self._watcher = remaining
 
-    def build_all(self, builder: 'Builder') -> None:
+    def build_all(
-        ''' Create virtual objects and build sources. '''
+        self,
-        self.make_once(builder)  # in case no page used the |vgroups filter
+        builder: 'Builder',
+        specific: Optional['GroupBySource'] = None
+    ) -> None:
+        '''
+        Build actual artifacts (if needed).
+        If `specific` is set, only build the artifacts for that single vobj
+        '''
+        if not self._watcher and not self._results:
+            return
+        with reporter.build('groupby', builder):  # type:ignore
+            # in case no page used the |vgroups filter
+            self.make_once([specific.config.key] if specific else None)
+            self._building = True
             path_cache = PathCache(builder.env)
             for vobj in self._results:
+                if specific and vobj.path != specific.path:
+                    continue
                 if vobj.slug:
                     builder.build(vobj, path_cache)
             del path_cache
+            self._building = False
             self._results.clear()  # garbage collect weak refs

lektor_groupby/model.py

@@ -48,6 +48,7 @@ class ModelReader:
         for r_key, subs in self._models.get(record.datamodel.id, {}).items():
             field = record[r_key]
             if not field:
+                yield FieldKeyPath(r_key), field
                 continue
             if subs == '*':  # either normal field or flow type (all blocks)
                 if self.flatten and isinstance(field, Flow):

lektor_groupby/plugin.py

@@ -7,7 +7,7 @@ from .pruner import prune
 from .resolver import Resolver
 from .vobj import VPATH, GroupBySource, GroupByBuildProgram
 if TYPE_CHECKING:
-    from lektor.builder import Builder, BuildState
+    from lektor.builder import Builder
     from lektor.sourceobj import SourceObject
     from .watcher import GroupByCallbackArgs
 
@@ -17,7 +17,6 @@ class GroupByPlugin(Plugin):
     description = 'Cluster arbitrary records with field attribute keyword.'
 
     def on_setup_env(self, **extra: Any) -> None:
-        self.has_changes = False
         self.resolver = Resolver(self.env)
         self.env.add_build_program(GroupBySource, GroupByBuildProgram)
         self.env.jinja_env.filters.update(vgroups=VGroups.iter)
@@ -30,19 +29,13 @@ class GroupByPlugin(Plugin):
         if isinstance(source, Asset):
             return
         groupby = self._init_once(builder)
-        if groupby.isNew and isinstance(source, GroupBySource):
+        if not groupby.isBuilding and isinstance(source, GroupBySource):
-            self.has_changes = True
+            # TODO: differentiate between actual build and browser preview
-
+            groupby.build_all(builder, source)
-    def on_after_build(self, build_state: 'BuildState', **extra: Any) -> None:
-        if build_state.updated_artifacts:
-            self.has_changes = True
 
     def on_after_build_all(self, builder: 'Builder', **extra: Any) -> None:
-        # only rebuild if has changes (bypass idle builds)
+        # by now, most likely already built. So, build_all() is a no-op
-        # or the very first time after startup (url resolver & pruning)
+        self._init_once(builder).build_all(builder)
-        if self.has_changes or not self.resolver.has_any:
-            self._init_once(builder).build_all(builder)  # updates resolver
-            self.has_changes = False
 
     def on_after_prune(self, builder: 'Builder', **extra: Any) -> None:
         # TODO: find a better way to prune unreferenced elements
@@ -78,7 +71,11 @@ class GroupByPlugin(Plugin):
             @watcher.grouping()
             def _fn(args: 'GroupByCallbackArgs') -> Iterator[str]:
                 val = args.field
-                if isinstance(val, str):
+                if isinstance(val, str) and val != '':
                     val = map(str.strip, val.split(split)) if split else [val]
+                elif isinstance(val, (bool, int, float)):
+                    val = [val]
+                elif not val:  # after checking for '', False, 0, and 0.0
+                    val = [None]
                 if isinstance(val, (list, map)):
                     yield from val

lektor_groupby/resolver.py

@@ -42,8 +42,12 @@ class Resolver:
     def files(self) -> Iterable[str]:
         return self._data
 
-    def reset(self) -> None:
+    def reset(self, optional_key: Optional[str] = None) -> None:
         ''' Clear previously recorded virtual objects. '''
+        if optional_key:
+            self._data = {k: v for k, v in self._data.items()
+                          if v.config.key != optional_key}
+        else:
             self._data.clear()
 
     def add(self, vobj: GroupBySource) -> None:

lektor_groupby/util.py

@@ -1,20 +1,9 @@
-from lektor.reporter import reporter, style
 from typing import List, Dict, Optional, TypeVar
 from typing import Callable, Any, Union, Generic
 
 T = TypeVar('T')
 
 
-def report_config_error(key: str, field: str, val: str, e: Exception) -> None:
-    ''' Send error message to Lektor reporter. Indicate which field is bad. '''
-    msg = '[ERROR] invalid config for [{}.{}] = "{}",  Error: {}'.format(
-        key, field, val, repr(e))
-    try:
-        reporter._write_line(style(msg, fg='red'))
-    except Exception:
-        print(msg)  # fallback in case Lektor API changes
-
-
 def most_used_key(keys: List[T]) -> Optional[T]:
     ''' Find string with most occurrences. '''
     if len(keys) < 3:
@@ -58,3 +47,16 @@ def build_url(parts: List[str]) -> str:
     if '.' not in url.rsplit('/', 1)[-1]:
         url += '/'
     return url or '/'
+
+
+class cached_property(Generic[T]):
+    ''' Calculate complex property only once. '''
+
+    def __init__(self, fn: Callable[[Any], T]) -> None:
+        self.fn = fn
+
+    def __get__(self, obj: object, typ: Union[type, None] = None) -> T:
+        if obj is None:
+            return self  # type: ignore
+        ret = obj.__dict__[self.fn.__name__] = self.fn(obj)
+        return ret

lektor_groupby/vobj.py

@@ -3,14 +3,11 @@ from lektor.context import get_ctx
 from lektor.db import _CmpHelper
 from lektor.environment import Expression
 from lektor.sourceobj import VirtualSourceObject  # subclass
-from werkzeug.utils import cached_property
+from typing import TYPE_CHECKING
-
+from typing import List, Any, Dict, Optional, Generator, Iterator, Iterable
-from typing import TYPE_CHECKING, List, Any, Optional, Iterator, Iterable
 from .pagination import PaginationConfig
 from .query import FixedRecordsQuery
-from .util import (
+from .util import most_used_key, insert_before_ext, build_url, cached_property
-    report_config_error, most_used_key, insert_before_ext, build_url
-)
 if TYPE_CHECKING:
     from lektor.pagination import Pagination
     from lektor.builder import Artifact
@@ -40,60 +37,64 @@ class GroupBySource(VirtualSourceObject):
         super().__init__(record)
         self.key = slug
         self.page_num = page_num
+        self._expr_fields = {}  # type: Dict[str, Expression]
         self.__children = []  # type: List[str]
-        self.__group_map = []  # type: List[str]
+        self.__group_map = []  # type: List[Any]
 
-    def append_child(self, child: 'Record', group: str) -> None:
+    def append_child(self, child: 'Record', group: Any) -> None:
         if child not in self.__children:
             self.__children.append(child.path)
+        # TODO: rename group to value
         # __group_map is later used to find most used group
         self.__group_map.append(group)
 
+    def _update_attr(self, key: str, value: Any) -> None:
+        ''' Set or remove Jinja evaluated Expression field. '''
+        if isinstance(value, Expression):
+            self._expr_fields[key] = value
+            try:
+                delattr(self, key)
+            except AttributeError:
+                pass
+        else:
+            if key in self._expr_fields:
+                del self._expr_fields[key]
+            setattr(self, key, value)
+
     # -------------------------
     #   Evaluate Extra Fields
     # -------------------------
 
-    def finalize(self, config: 'Config', group: Optional[str] = None) \
+    def finalize(self, config: 'Config', group: Optional[Any] = None) \
             -> 'GroupBySource':
         self.config = config
         # make a sorted children query
         self._query = FixedRecordsQuery(self.pad, self.__children, self.alt)
         self._query._order_by = config.order_by
+        del self.__children
         # set group name
         self.group = group or most_used_key(self.__group_map)
-        # cleanup temporary data
-        del self.__children
         del self.__group_map
         # evaluate slug Expression
-        self.slug = None  # type: Optional[str]
+        self.slug = config.eval_slug(self.key, on=self)
-        if config.slug and '{key}' in config.slug:
-            self.slug = config.slug.replace('{key}', self.key)
-        else:
-            self.slug = self._eval(config.slug, field='slug')
-            assert self.slug != Ellipsis, 'invalid config: ' + config.slug
         if self.slug and self.slug.endswith('/index.html'):
             self.slug = self.slug[:-10]
-        # extra fields
-        for attr, expr in config.fields.items():
-            setattr(self, attr, self._eval(expr, field='fields.' + attr))
-        return self
 
-    def _eval(self, value: Any, *, field: str) -> Any:
+        if group:  # exit early if initialized through resolver
-        ''' Internal only: evaluates Lektor config file field expression. '''
+            return self
-        if not isinstance(value, str):
+        # extra fields
-            return value
+        for attr in config.fields:
-        pad = self.record.pad
+            self._update_attr(attr, config.eval_field(attr, on=self))
-        alt = self.record.alt
+        return self
-        try:
-            return Expression(pad.env, value).evaluate(pad, this=self, alt=alt)
-        except Exception as e:
-            report_config_error(self.config.key, field, value, e)
-            return Ellipsis
 
     # -----------------------
     #   Pagination handling
     # -----------------------
 
+    @property
+    def supports_pagination(self) -> bool:
+        return self.config.pagination['enabled']  # type: ignore[no-any-return]
+
     @cached_property
     def _pagination_config(self) -> 'PaginationConfig':
         # Generate `PaginationConfig` once we need it
@@ -128,25 +129,30 @@ class GroupBySource(VirtualSourceObject):
             vpath += '/' + str(self.page_num)
         return vpath
 
-    @property
+    @cached_property
-    def url_path(self) -> str:
+    def url_path(self) -> str:  # type: ignore[override]
-        # Actual path to resource as seen by the browser
+        ''' Actual path to resource as seen by the browser. '''
+        # check if slug is absolute URL
+        slug = self.slug
+        if slug and slug.startswith('/'):
+            parts = [self.pad.get_root(alt=self.alt).url_path]
+        else:
             parts = [self.record.url_path]
         # slug can be None!!
-        if not self.slug:
+        if not slug:
             return build_url(parts)
         # if pagination enabled, append pagination.url_suffix to path
         if self.page_num and self.page_num > 1:
             sffx = self._pagination_config.url_suffix
-            if '.' in self.slug.split('/')[-1]:
+            if '.' in slug.rsplit('/', 1)[-1]:
                 # default: ../slugpage2.html (use e.g.: url_suffix = .page.)
                 parts.append(insert_before_ext(
-                    self.slug, sffx + str(self.page_num), '.'))
+                    slug, sffx + str(self.page_num), '.'))
             else:
                 # default: ../slug/page/2/index.html
-                parts += [self.slug, sffx, self.page_num]
+                parts += [slug, sffx, self.page_num]
         else:
-            parts.append(self.slug)
+            parts.append(slug)
         return build_url(parts)
 
     def iter_source_filenames(self) -> Generator[str, None, None]:
@@ -188,9 +194,23 @@ class GroupBySource(VirtualSourceObject):
             return getattr(self, key[1:])
         return self.__missing__(key)
 
+    def __getattr__(self, key: str) -> Any:
+        ''' Lazy evaluate custom user field expressions. '''
+        if key in self._expr_fields:
+            expr = self._expr_fields[key]
+            return expr.evaluate(self.pad, this=self, alt=self.alt)
+        raise AttributeError
+
     def __lt__(self, other: 'GroupBySource') -> bool:
         # Used for |sort filter ("group" is the provided original string)
-        return self.group.lower() < other.group.lower()
+        if isinstance(self.group, (bool, int, float)) and \
+                isinstance(other.group, (bool, int, float)):
+            return self.group < other.group
+        if self.group is None:
+            return False
+        if other.group is None:
+            return True
+        return str(self.group).lower() < str(other.group).lower()
 
     def __eq__(self, other: object) -> bool:
         # Used for |unique filter

lektor_groupby/watcher.py

@@ -1,4 +1,4 @@
-from typing import TYPE_CHECKING, Dict, List, Tuple, Any, Union, NamedTuple
+from typing import TYPE_CHECKING, Dict, List, Any, Union, NamedTuple
 from typing import Optional, Callable, Iterator, Generator
 from .backref import VGroups
 from .model import ModelReader
@@ -16,8 +16,8 @@ class GroupByCallbackArgs(NamedTuple):
 
 
 GroupingCallback = Callable[[GroupByCallbackArgs], Union[
-    Iterator[Union[str, Tuple[str, Any]]],
+    Iterator[Any],
-    Generator[Union[str, Tuple[str, Any]], Optional[str], None],
+    Generator[Optional[str], Any, None],
 ]]
 
 
@@ -49,7 +49,8 @@ class Watcher:
         assert callable(self.callback), 'No grouping callback provided.'
         self._model_reader = ModelReader(pad.db, self.config.key, self.flatten)
         self._root_record = {}  # type: Dict[str, Record]
-        self._state = {}  # type: Dict[str, Dict[str, GroupBySource]]
+        self._state = {}  # type: Dict[str, Dict[Optional[str], GroupBySource]]
+        self._rmmbr = []  # type: List[Record]
         for alt in pad.config.iter_alternatives():
             self._root_record[alt] = pad.get(self._root, alt=alt)
             self._state[alt] = {}
@@ -64,13 +65,15 @@ class Watcher:
         Each record is guaranteed to be processed only once.
         '''
         for key, field in self._model_reader.read(record):
-            _gen = self.callback(GroupByCallbackArgs(record, key, field))
+            args = GroupByCallbackArgs(record, key, field)
+            _gen = self.callback(args)
             try:
                 group = next(_gen)
                 while True:
-                    if not isinstance(group, str):
+                    if self.config.key_map_fn:
-                        raise TypeError(f'Unsupported groupby yield: {group}')
+                        slug = self._persist_multiple(args, group)
-                    slug = self._persist(record, key, group)
+                    else:
+                        slug = self._persist(args, group)
                     # return slugified group key and continue iteration
                     if isinstance(_gen, Generator) and not _gen.gi_yieldfrom:
                         group = _gen.send(slug)
@@ -79,24 +82,57 @@ class Watcher:
             except StopIteration:
                 del _gen
 
-    def _persist(self, record: 'Record', key: 'FieldKeyPath', group: str) \
+    def _persist_multiple(self, args: 'GroupByCallbackArgs', obj: Any) \
-            -> str:
+            -> Optional[str]:
+        # if custom key mapping function defined, use that first
+        res = self.config.eval_key_map_fn(on=args.record,
+                                          context={'X': obj, 'SRC': args})
+        if isinstance(res, (list, tuple)):
+            for k in res:
+                self._persist(args, k)  # 1-to-n replacement
+            return None
+        return self._persist(args, res)  # normal & null replacement
+
+    def _persist(self, args: 'GroupByCallbackArgs', obj: Any) \
+            -> Optional[str]:
         ''' Update internal state. Return slugified string. '''
-        alt = record.alt
+        if not isinstance(obj, (str, bool, int, float)) and obj is not None:
-        slug = self.config.slugify(group)
+            raise ValueError(
+                'Unsupported groupby yield type for [{}]:'
+                ' {} (expected str, got {})'.format(
+                    self.config.key, obj, type(obj).__name__))
+
+        if obj is None:
+            # if obj is not set, test if config.replace_none_key is set
+            slug = self.config.replace_none_key
+            obj = slug
+        else:
+            # if obj is set, apply config.key_map  (convert int -> str)
+            slug = self.config.slugify(str(obj)) or None
+        # if neither custom mapping succeeded, do not process further
+        if not slug or obj is None:
+            return slug
+        # update internal object storage
+        alt = args.record.alt
         if slug not in self._state[alt]:
             src = GroupBySource(self._root_record[alt], slug)
             self._state[alt][slug] = src
         else:
             src = self._state[alt][slug]
 
-        src.append_child(record, group)
+        src.append_child(args.record, obj)  # obj is used as "group" string
         # reverse reference
-        VGroups.of(record).add(key, src)
+        VGroups.of(args.record).add(args.key, src)
         return slug
 
-    def iter_sources(self, root: 'Record') -> Iterator[GroupBySource]:
+    def remember(self, record: 'Record') -> None:
+        self._rmmbr.append(record)
+
+    def iter_sources(self) -> Iterator[GroupBySource]:
         ''' Prepare and yield GroupBySource elements. '''
+        for x in self._rmmbr:
+            self.process(x)
+        del self._rmmbr
         for vobj_list in self._state.values():
             for vobj in vobj_list.values():
                 yield vobj.finalize(self.config)