backend/app/services/importer.py

@@ -35,6 +35,7 @@ from ..utils import safe_probe
 from ..utils.paths import (
     derive_subdir,
     derive_top_level_artist,
+    canonical_subdir,
     filehash_from_url,
     hash_suffixed_name,
     safe_ext,
@@ -944,7 +945,7 @@ class Importer:
                 )
                 return ImportResult(status="superseded", image_id=match_id)
 
-        dest = self._copy_to_library(source, sha, attribution_path)
+        dest = self._copy_to_library(source, sha, attribution_path, path_artist)
 
         record = ImageRecord(
             path=str(dest),
@@ -1600,7 +1601,8 @@ class Importer:
         )
 
     def _copy_to_library(
-        self, source: Path, sha: str, attribution_path: Path
+        self, source: Path, sha: str, attribution_path: Path,
+        artist: Artist | None = None,
     ) -> Path:
         """Copy `source` to its final library path. Returns the destination.
 
@@ -1608,8 +1610,18 @@ class Importer:
         _import_media (filesystem scan) and _supersede (when new_path is
         not passed). FC-3c's attach_in_place skips this helper entirely
         — the file is already at its final home.
+
+        `artist`, when resolved, decides the top-level directory: the
+        library is keyed on the Artist row's slug, NOT on however the
+        import folder happened to be capitalised. Without that, an import
+        from `/import/Conto/` and a download for the same artist write to
+        `Conto/` and `conto/` respectively and the library grows a second
+        home for one artist (milestone #421).
         """
-        subdir = derive_subdir(attribution_path, self.import_root)
+        subdir = canonical_subdir(
+            derive_subdir(attribution_path, self.import_root),
+            artist.slug if artist else None,
+        )
         dest_dir = self.images_root / subdir if subdir else self.images_root
         dest_dir.mkdir(parents=True, exist_ok=True)
         dest_name = hash_suffixed_name(source.stem, sha, source.suffix)
@@ -1644,7 +1656,16 @@ class Importer:
         that path (FC-3c attach_in_place case) — skip the copy step.
         Otherwise the file is copied via _copy_to_library."""
         if new_path is None:
-            dest = self._copy_to_library(source, sha, source)
+            # The KEPT row's artist decides the destination, not the incoming
+            # file's folder — a supersede rewrites `existing.path`, so writing
+            # it anywhere but that artist's canonical directory would move a
+            # row OUT of the tree milestone #421 is consolidating. ImageRecord
+            # carries `artist_id` with no relationship attribute, so this is a
+            # lookup rather than `existing.artist`.
+            kept_artist = artist
+            if kept_artist is None and existing.artist_id is not None:
+                kept_artist = self.session.get(Artist, existing.artist_id)
+            dest = self._copy_to_library(source, sha, source, kept_artist)
         else:
             dest = new_path
 

backend/app/utils/paths.py

@@ -60,6 +60,37 @@ def derive_subdir(source_path: Path, import_root: Path) -> str:
     return str(rel) if str(rel) != "." else ""
 
 
+def canonical_subdir(subdir: str, artist_slug: str | None) -> str:
+    """`subdir` with its TOP-LEVEL segment replaced by the artist's slug.
+
+        canonical_subdir("Conto/patreon", "conto")  -> "conto/patreon"
+        canonical_subdir("Conto", "conto")          -> "conto"
+        canonical_subdir("Conto/patreon", None)     -> "Conto/patreon"
+        canonical_subdir("", "conto")               -> ""
+
+    `derive_subdir` mirrors the IMPORT tree's folder names verbatim, so a
+    filesystem import out of `/import/Conto/...` used to write
+    `<images_root>/Conto/...` while the download path wrote `<root>/conto/...`
+    for the very same Artist row. One artist, two directories, forever — 57
+    such families had accumulated by 2026-09-21, and the database never had
+    duplicate artists at all (milestone #421).
+
+    The slug is the canonical name because it is the Artist row's own
+    identifier: it is what `/api/artists` reports, what the ingesters already
+    write, and the one spelling that cannot vary with how a folder happened to
+    be capitalised on the way in.
+
+    Two deliberate pass-throughs. NO artist resolved means there is nothing
+    authoritative to canonicalise against, and an EMPTY subdir is a file
+    landing at the images root — those have no artist folder to correct, and
+    what becomes of them is its own decision (task #4247), not a side effect
+    of this helper.
+    """
+    if not artist_slug or not subdir:
+        return subdir
+    return str(Path(artist_slug, *Path(subdir).parts[1:]))
+
+
 def hash_suffixed_name(stem: str, sha256_hex: str, ext: str) -> str:
     """Builds 'stem__<first10ofhash><ext>'.
 

tests/test_importer.py

@@ -58,10 +58,36 @@ def test_import_one_happy_path(importer, import_layout):
     result = importer.import_one(src)
     assert result.status == "imported"
     record = importer.session.get(ImageRecord, result.image_id)
-    assert record.path.startswith(str(images_root / "Alice"))
+    # Under the artist's SLUG, not the import folder's "Alice" (milestone
+    # #421) — the import tree's capitalisation is an accident of how the
+    # operator named a folder, and honouring it gave one artist two homes.
+    assert record.path.startswith(str(images_root / "alice"))
     assert record.path.endswith(".jpg")
 
 
+def test_library_path_uses_the_artist_slug_not_the_folder_name(
+    importer, import_layout,
+):
+    """The regression that grew 57 duplicate artist directories: an import
+    from `/import/Conto/...` wrote `<images>/Conto/...` while the downloader
+    wrote `<images>/conto/...` for the same Artist row."""
+    import_root, images_root = import_layout
+    src = import_root / "StickySpoodge" / "patreon" / "a.jpg"
+    _make_jpeg(src)
+    result = importer.import_one(src)
+
+    record = importer.session.get(ImageRecord, result.image_id)
+    artist = importer.session.execute(
+        select(Artist).where(Artist.slug == "stickyspoodge")
+    ).scalar_one()
+    assert artist.name == "StickySpoodge"
+    # Artist segment canonicalised; the post hierarchy below it untouched.
+    assert record.path.startswith(
+        str(images_root / "stickyspoodge" / "patreon")
+    )
+    assert not Path(record.path).is_relative_to(images_root / "StickySpoodge")
+
+
 def test_folder_creates_artist_and_links_artist_id(importer, import_layout):
     # FC-2d-vii-c: folder import creates the Artist and sets the canonical
     # image_record.artist_id — no artist-kind Tag (that path was retired;

tests/test_paths.py

@@ -1,6 +1,7 @@
 from pathlib import Path
 
 from backend.app.utils.paths import (
+    canonical_subdir,
     derive_subdir,
     derive_top_level_artist,
     filehash_from_url,
@@ -59,3 +60,38 @@ def test_derive_top_level_artist_nested():
 
 def test_derive_top_level_artist_root():
     assert derive_top_level_artist(Path("/import/x.png"), IMPORT) is None
+
+
+# --- canonical_subdir (milestone #421) --------------------------------------
+
+
+def test_canonical_subdir_replaces_the_top_segment():
+    assert canonical_subdir("Conto/patreon", "conto") == "conto/patreon"
+    assert canonical_subdir("Conto", "conto") == "conto"
+
+
+def test_canonical_subdir_keeps_everything_below_the_artist():
+    """Only the artist segment is authoritative — post folders below it are
+    the downloader's business and must survive untouched."""
+    assert canonical_subdir(
+        "Big Bang/patreon/2026-01-02_123_A Post", "big-bang"
+    ) == "big-bang/patreon/2026-01-02_123_A Post"
+
+
+def test_canonical_subdir_passes_through_without_an_artist():
+    """No resolved artist means nothing authoritative to canonicalise against,
+    so the import folder's own name stands."""
+    assert canonical_subdir("Conto/patreon", None) == "Conto/patreon"
+    assert canonical_subdir("Conto", "") == "Conto"
+
+
+def test_canonical_subdir_leaves_the_images_root_alone():
+    """An empty subdir is a file landing at the root — there is no artist
+    folder to correct, and what becomes of those is task #4247's call."""
+    assert canonical_subdir("", "conto") == ""
+    assert canonical_subdir("", None) == ""
+
+
+def test_canonical_subdir_is_idempotent():
+    once = canonical_subdir("Conto/patreon", "conto")
+    assert canonical_subdir(once, "conto") == once