move string length function to separate module

migrations/dimensions-config/1fae088c80b6.py

@@ -17,6 +17,7 @@
 from alembic import op
 from lsst.daf.butler import Timespan
 from lsst.daf.butler_migrate.butler_attributes import ButlerAttributes
+from lsst.daf.butler_migrate.naming import make_string_length_constraint
 from lsst.daf.butler_migrate.timespan import create_timespan_column_definitions, format_timespan_value
 from lsst.utils import doImportType
 
@@ -98,7 +99,7 @@ def _migrate_groups(ctx: _Context) -> None:
     _LOG.info("Creating group table")
     check_constraints = []
     if ctx.is_sqlite:
-        check_constraints = [_make_string_check_constraint("instrument", 32, "group_len_instrument")]
+        check_constraints = [make_string_length_constraint("instrument", 32, "group_len_instrument")]
     table = op.create_table(
         "group",
         sa.Column("instrument", sa.String(32), primary_key=True),

python/lsst/daf/butler_migrate/naming.py

@@ -32,13 +32,13 @@
 
 from typing import TYPE_CHECKING
 
+import sqlalchemy
+
 from .shrink import shrinkDatabaseEntityName
 
 if TYPE_CHECKING:
     from collections.abc import Iterable
 
-    import sqlalchemy
-
 
 def primary_key_name(table: str, bind: sqlalchemy.engine.Connection) -> str:
     """Return name of a primary key constraint for a table.
@@ -180,3 +180,31 @@ def is_foreign_key_index(table: str, index_name: str) -> bool:
 
 def is_regular_index(table: str, index_name: str) -> bool:
     return index_name.startswith(f"{table}_idx_")
+
+
+def make_string_length_constraint(
+    column_name: str, max_length: int, constraint_name: str
+) -> sqlalchemy.schema.CheckConstraint:
+    """Create a check constraint that guarantees a string column has a length
+    that is non-zero and less than a specified maximum.
+
+    These constraints are used by Butler in sqlite databases to emulate
+    VARCHARs with a specific length.
+
+    Parameters
+    ----------
+    column_name : `str`
+        The name of the column to create the constraint on.
+    max_length : `int`
+        The maximum length allowed for strings stored in this column.
+    constraint_name : `str`
+        An arbitrary identifier for the constraint.
+
+    Returns
+    -------
+    check_constraint : `sqlalchemy.schema.CheckConstraint`
+        The generated check constraint.
+    """
+    return sqlalchemy.schema.CheckConstraint(
+        f'length("{column_name}")<={max_length} AND length("{column_name}")>=1', name=constraint_name
+    )