Improve TAG field sortability error message and documentation

- Update error message to clearly explain that TAG fields cannot be sortable
- Add explanation that string fields default to TAG fields for exact matching
- Provide solution to use full_text_search=True for sortable TEXT fields
- Document field type mapping and sortability rules in models.md and errors.md

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: abrookins

aredis_om/model/model.py

@@ -2906,9 +2906,10 @@ def schema_for_type(
             case_sensitive = getattr(field_info, "case_sensitive", False)
             full_text_search = getattr(field_info, "full_text_search", False)
             sortable_tag_error = RedisModelError(
-                "In this Preview release, TAG fields cannot "
-                f"be marked as sortable. Problem field: {name}. "
-                "See docs: TODO"
+                f"TAG fields cannot be marked as sortable. Problem field: {name}. "
+                f"String fields are indexed as TAG fields by default, which only support exact matching. "
+                f"To make this field sortable, add 'full_text_search=True' to create a TEXT field instead: "
+                f"Field(index=True, sortable=True, full_text_search=True)"
             )
 
             # For more complicated compound validators (e.g. PositiveInt), we might get a _GenericAlias rather than

docs/errors.md

@@ -38,6 +38,13 @@ class Member(JsonModel):
 
 **NOTE:** Only an indexed field can be sortable.
 
+**IMPORTANT:** String fields are indexed as TAG fields by default, which cannot be sortable. Only NUMERIC, TEXT, and GEO field types support sorting. To make a string field sortable, you must add `full_text_search=True` to create a TEXT field:
+
+```python
+class Member(JsonModel):
+    name: str = Field(index=True, sortable=True, full_text_search=True)
+```
+
 ## E3
 
 >You tried to do a full-text search on the field '{field.name}', but the field is not indexed for full-text search. Use the full_text_search=True option.

docs/models.md

@@ -250,6 +250,31 @@ class Customer(HashModel):
 
 In this example, we marked `Customer.last_name` as indexed.
 
+### Field Index Types
+
+Redis OM automatically chooses the appropriate RediSearch field type based on the Python field type and options:
+
+- **String fields** → **TAG fields** by default (exact matching only), or **TEXT fields** if `full_text_search=True`
+- **Numeric fields** (int, float) → **NUMERIC fields** (range queries and sorting)
+- **Boolean fields** → **TAG fields**
+- **Datetime fields** → **NUMERIC fields** (stored as Unix timestamps)
+- **Geographic fields** → **GEO fields**
+
+### Making String Fields Sortable
+
+By default, string fields are indexed as TAG fields, which only support exact matching and cannot be sorted. To make a string field sortable, you must create a TEXT field by adding `full_text_search=True`:
+
+```python
+class Customer(HashModel):
+    # TAG field - exact matching only, cannot be sorted
+    category: str = Field(index=True)
+    
+    # TEXT field - supports full-text search and sorting
+    name: str = Field(index=True, sortable=True, full_text_search=True)
+```
+
+Only NUMERIC, TEXT, and GEO field types support sorting in RediSearch.
+
 To create the indexes for any models that have indexed fields, use the `migrate` CLI command that Redis OM installs in your Python environment.
 
 This command detects any `JsonModel` or `HashModel` instances in your project and does the following for each model that isn't abstract or embedded: