Recreate indexes when calling transform when possible and raise an er…

docs/python-api.rst

@@ -1402,6 +1402,8 @@ To keep the original table around instead of dropping it, pass the ``keep_table=
 
     table.transform(types={"age": int}, keep_table="original_table")
 
+This method raises a ``sqlite_utils.db.TransformError`` exception if the table cannot be transformed, usually because there are existing constraints or indexes that are incompatible with modifications to the columns.
+
 .. _python_api_transform_alter_column_types:
 
 Altering column types

sqlite_utils/db.py

@@ -156,6 +156,10 @@
 Trigger = namedtuple("Trigger", ("name", "table", "sql"))
 
 
+class TransformError(Exception):
+    pass
+
+
 ForeignKeyIndicator = Union[
     str,
     ForeignKey,
@@ -1967,6 +1971,30 @@ def transform_sql(
         sqls.append(
             "ALTER TABLE [{}] RENAME TO [{}];".format(new_table_name, self.name)
         )
+        # Re-add existing indexes
+        for index in self.indexes:
+            if index.origin != "pk":
+                index_sql = self.db.execute(
+                    """SELECT sql FROM sqlite_master WHERE type = 'index' AND name = :index_name;""",
+                    {"index_name": index.name},
+                ).fetchall()[0][0]
+                if index_sql is None:
+                    raise TransformError(
+                        f"Index '{index.name}' on table '{self.name}' does not have a "
+                        "CREATE INDEX statement. You must manually drop this index prior to running this "
+                        "transformation and manually recreate the new index after running this transformation."
+                    )
+                if keep_table:
+                    sqls.append(f"DROP INDEX IF EXISTS [{index.name}];")
+                for col in index.columns:
+                    if col in rename.keys() or col in drop:
+                        raise TransformError(
+                            f"Index '{index.name}' column '{col}' is not in updated table '{self.name}'. "
+                            f"You must manually drop this index prior to running this transformation "
+                            f"and manually recreate the new index after running this transformation. "
+                            f"The original index sql statement is: `{index_sql}`. No changes have been applied to this table."
+                        )
+                sqls.append(index_sql)
         return sqls
 
     def extract(

tests/test_transform.py

@@ -1,4 +1,4 @@
-from sqlite_utils.db import ForeignKey
+from sqlite_utils.db import ForeignKey, TransformError
 from sqlite_utils.utils import OperationalError
 import pytest
 
@@ -539,3 +539,125 @@ def test_transform_strict(fresh_db, strict):
     assert dogs.strict == strict or not fresh_db.supports_strict
     dogs.transform(not_null={"name"})
     assert dogs.strict == strict or not fresh_db.supports_strict
+
+
+@pytest.mark.parametrize(
+    "indexes, transform_params",
+    [
+        ([["name"]], {"types": {"age": str}}),
+        ([["name"], ["age", "breed"]], {"types": {"age": str}}),
+        ([], {"types": {"age": str}}),
+        ([["name"]], {"types": {"age": str}, "keep_table": "old_dogs"}),
+    ],
+)
+def test_transform_indexes(fresh_db, indexes, transform_params):
+    # https://github.com/simonw/sqlite-utils/issues/633
+    # New table should have same indexes as old table after transformation
+    dogs = fresh_db["dogs"]
+    dogs.insert({"id": 1, "name": "Cleo", "age": 5, "breed": "Labrador"}, pk="id")
+
+    for index in indexes:
+        dogs.create_index(index)
+
+    indexes_before_transform = dogs.indexes
+
+    dogs.transform(**transform_params)
+
+    assert sorted(
+        [
+            {k: v for k, v in idx._asdict().items() if k != "seq"}
+            for idx in dogs.indexes
+        ],
+        key=lambda x: x["name"],
+    ) == sorted(
+        [
+            {k: v for k, v in idx._asdict().items() if k != "seq"}
+            for idx in indexes_before_transform
+        ],
+        key=lambda x: x["name"],
+    ), f"Indexes before transform: {indexes_before_transform}\nIndexes after transform: {dogs.indexes}"
+    if "keep_table" in transform_params:
+        assert all(
+            index.origin == "pk"
+            for index in fresh_db[transform_params["keep_table"]].indexes
+        )
+
+
+def test_transform_retains_indexes_with_foreign_keys(fresh_db):
+    dogs = fresh_db["dogs"]
+    owners = fresh_db["owners"]
+
+    dogs.insert({"id": 1, "name": "Cleo", "owner_id": 1}, pk="id")
+    owners.insert({"id": 1, "name": "Alice"}, pk="id")
+
+    dogs.create_index(["name"])
+
+    indexes_before_transform = dogs.indexes
+
+    fresh_db.add_foreign_keys([("dogs", "owner_id", "owners", "id")])  # calls transform
+
+    assert sorted(
+        [
+            {k: v for k, v in idx._asdict().items() if k != "seq"}
+            for idx in dogs.indexes
+        ],
+        key=lambda x: x["name"],
+    ) == sorted(
+        [
+            {k: v for k, v in idx._asdict().items() if k != "seq"}
+            for idx in indexes_before_transform
+        ],
+        key=lambda x: x["name"],
+    ), f"Indexes before transform: {indexes_before_transform}\nIndexes after transform: {dogs.indexes}"
+
+
+@pytest.mark.parametrize(
+    "transform_params",
+    [
+        {"rename": {"age": "dog_age"}},
+        {"drop": ["age"]},
+    ],
+)
+def test_transform_with_indexes_errors(fresh_db, transform_params):
+    # Should error with a compound (name, age) index if age is renamed or dropped
+    dogs = fresh_db["dogs"]
+    dogs.insert({"id": 1, "name": "Cleo", "age": 5}, pk="id")
+
+    dogs.create_index(["name", "age"])
+
+    with pytest.raises(TransformError) as excinfo:
+        dogs.transform(**transform_params)
+
+    assert (
+        "Index 'idx_dogs_name_age' column 'age' is not in updated table 'dogs'. "
+        "You must manually drop this index prior to running this transformation"
+        in str(excinfo.value)
+    )
+
+
+def test_transform_with_unique_constraint_implicit_index(fresh_db):
+    dogs = fresh_db["dogs"]
+    # Create a table with a UNIQUE constraint on 'name', which creates an implicit index
+    fresh_db.execute(
+        """
+        CREATE TABLE dogs (
+            id INTEGER PRIMARY KEY,
+            name TEXT UNIQUE,
+            age INTEGER
+        );
+    """
+    )
+    dogs.insert({"id": 1, "name": "Cleo", "age": 5})
+
+    # Attempt to transform the table without modifying 'name'
+    with pytest.raises(TransformError) as excinfo:
+        dogs.transform(types={"age": str})
+
+    assert (
+        "Index 'sqlite_autoindex_dogs_1' on table 'dogs' does not have a CREATE INDEX statement."
+        in str(excinfo.value)
+    )
+    assert (
+        "You must manually drop this index prior to running this transformation and manually recreate the new index after running this transformation."
+        in str(excinfo.value)
+    )