feat: add faceted search in CrudFactory (#97)

* feat: add faceted search in CrudFactory

* feat: add filter_params_schema in CrudFactory

* fix: add missing Raises in build_search_filters docstring

* fix: faceted search

* fix: cov

* fix: documentation/filter_params

src/fastapi_toolsets/crud/search.py

@@ -1,19 +1,23 @@
 """Search utilities for AsyncCrud."""
 
+import asyncio
+from collections import Counter
 from collections.abc import Sequence
 from dataclasses import dataclass
 from typing import TYPE_CHECKING, Any, Literal
 
-from sqlalchemy import String, or_
+from sqlalchemy import String, or_, select
+from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.orm import DeclarativeBase
 from sqlalchemy.orm.attributes import InstrumentedAttribute
 
-from ..exceptions import NoSearchableFieldsError
+from ..exceptions import InvalidFacetFilterError, NoSearchableFieldsError
 
 if TYPE_CHECKING:
     from sqlalchemy.sql.elements import ColumnElement
 
 SearchFieldType = InstrumentedAttribute[Any] | tuple[InstrumentedAttribute[Any], ...]
+FacetFieldType = SearchFieldType
 
 
 @dataclass
@@ -89,6 +93,9 @@ def build_search_filters(
 
     Returns:
         Tuple of (filter_conditions, joins_needed)
+
+    Raises:
+        NoSearchableFieldsError: If no searchable field has been configured
     """
     # Normalize input
     if isinstance(search, str):
@@ -136,7 +143,7 @@ def build_search_filters(
         else:
             filters.append(column_as_string.ilike(f"%{query}%"))
 
-    if not filters:
+    if not filters:  # pragma: no cover
         return [], []
 
     # Combine based on match_mode
@@ -144,3 +151,145 @@ def build_search_filters(
         return [or_(*filters)], joins
     else:
         return filters, joins
+
+
+def facet_keys(facet_fields: Sequence[FacetFieldType]) -> list[str]:
+    """Return a key for each facet field, disambiguating duplicate column keys.
+
+    Args:
+        facet_fields: Sequence of facet fields — either direct columns or
+            relationship tuples ``(rel, ..., column)``.
+
+    Returns:
+        A list of string keys, one per facet field, in the same order.
+    """
+    raw: list[tuple[str, str | None]] = []
+    for field in facet_fields:
+        if isinstance(field, tuple):
+            rel = field[-2]
+            column = field[-1]
+            raw.append((column.key, rel.key))
+        else:
+            raw.append((field.key, None))
+
+    counts = Counter(col_key for col_key, _ in raw)
+    keys: list[str] = []
+    for col_key, rel_key in raw:
+        if counts[col_key] > 1 and rel_key is not None:
+            keys.append(f"{rel_key}__{col_key}")
+        else:
+            keys.append(col_key)
+    return keys
+
+
+async def build_facets(
+    session: "AsyncSession",
+    model: type[DeclarativeBase],
+    facet_fields: Sequence[FacetFieldType],
+    *,
+    base_filters: "list[ColumnElement[bool]] | None" = None,
+    base_joins: list[InstrumentedAttribute[Any]] | None = None,
+) -> dict[str, list[Any]]:
+    """Return distinct values for each facet field, respecting current filters.
+
+    Args:
+        session: DB async session
+        model: SQLAlchemy model class
+        facet_fields: Columns or relationship tuples to facet on
+        base_filters: Filter conditions already applied to the main query (search + caller filters)
+        base_joins: Relationship joins already applied to the main query
+
+    Returns:
+        Dict mapping column key to sorted list of distinct non-None values
+    """
+    existing_join_keys: set[str] = {str(j) for j in (base_joins or [])}
+
+    keys = facet_keys(facet_fields)
+
+    async def _query_facet(field: FacetFieldType, key: str) -> tuple[str, list[Any]]:
+        if isinstance(field, tuple):
+            # Relationship chain: (User.role, Role.name) — last element is the column
+            rels = field[:-1]
+            column = field[-1]
+        else:
+            rels = ()
+            column = field
+
+        q = select(column).select_from(model).distinct()
+
+        # Apply base joins (already done on main query, but needed here independently)
+        for rel in base_joins or []:
+            q = q.outerjoin(rel)
+
+        # Add any extra joins required by this facet field that aren't already in base_joins
+        for rel in rels:
+            if str(rel) not in existing_join_keys:
+                q = q.outerjoin(rel)
+
+        if base_filters:
+            from sqlalchemy import and_
+
+            q = q.where(and_(*base_filters))
+
+        q = q.order_by(column)
+        result = await session.execute(q)
+        values = [row[0] for row in result.all() if row[0] is not None]
+        return key, values
+
+    pairs = await asyncio.gather(
+        *[_query_facet(f, k) for f, k in zip(facet_fields, keys)]
+    )
+    return dict(pairs)
+
+
+def build_filter_by(
+    filter_by: dict[str, Any],
+    facet_fields: Sequence[FacetFieldType],
+) -> tuple["list[ColumnElement[bool]]", list[InstrumentedAttribute[Any]]]:
+    """Translate a {column_key: value} dict into SQLAlchemy filter conditions.
+
+    Args:
+        filter_by: Mapping of column key to scalar value or list of values
+        facet_fields: Declared facet fields to validate keys against
+
+    Returns:
+        Tuple of (filter_conditions, joins_needed)
+
+    Raises:
+        InvalidFacetFilterError: If a key in filter_by is not a declared facet field
+    """
+    index: dict[
+        str, tuple[InstrumentedAttribute[Any], list[InstrumentedAttribute[Any]]]
+    ] = {}
+    for key, field in zip(facet_keys(facet_fields), facet_fields):
+        if isinstance(field, tuple):
+            rels = list(field[:-1])
+            column = field[-1]
+        else:
+            rels = []
+            column = field
+        index[key] = (column, rels)
+
+    valid_keys = set(index)
+    filters: list[ColumnElement[bool]] = []
+    joins: list[InstrumentedAttribute[Any]] = []
+    added_join_keys: set[str] = set()
+
+    for key, value in filter_by.items():
+        if key not in index:
+            raise InvalidFacetFilterError(key, valid_keys)
+
+        column, rels = index[key]
+
+        for rel in rels:
+            rel_key = str(rel)
+            if rel_key not in added_join_keys:
+                joins.append(rel)
+                added_join_keys.add(rel_key)
+
+        if isinstance(value, list):
+            filters.append(column.in_(value))
+        else:
+            filters.append(column == value)
+
+    return filters, joins