MySql & Sqlalchemy - not all foreign keys visible - mysql

Question

The list of foreign keys is empty for one of my tables in MySql Workbench. When I try to add the missing FK's I get an error of this structure:
Executing:
ALTER TABLE <schema>.<table>
ADD INDEX <index_name> (<column_name> ASC);
ALTER TABLE <schema>.<table_name>
ADD CONSTRAINT <constraint_name>
FOREIGN KEY (<column_name>)
REFERENCES <schema>.<ref_table_name> (<ref_column_name>)
ON DELETE CASCADE
ON UPDATE CASCADE;
Operation failed: There was an error while applying the SQL script to the database.
ERROR 1452: Cannot add or update a child row: a foreign key constraint fails (<schema>.`#sql-49f_6`, CONSTRAINT <constraint_name> FOREIGN KEY (<column_name>) REFERENCES <ref_table_name> (<ref_column_name>) ON DELETE CASCADE ON UPDATE CASCADE)
SQL Statement:
ALTER TABLE <schema>.<table_name>
ADD CONSTRAINT <constraint_name>
FOREIGN KEY (<column_name>)
REFERENCES <schema>.<ref_table_name> (<ref_column_name>)
ON DELETE CASCADE
ON UPDATE CASCADE
Obviously, the foreign key I am trying to create seems to be in the database already. Some research on the web lead me to believe that in some cases MySql Workbench does not list all foreign keys (for various reasons), which might be true for my setup as well. Thus, I tried listing all foreign keys on the table I am trying modify using this query:
SELECT *
FROM
information_schema.KEY_COLUMN_USAGE
WHERE
constraint_schema = <schema> AND table_name = <table_name> AND
referenced_table_name IS NOT NULL;
The result set is as empty as the Workbench foreign key list...
Looking at the error message I seem to have a weird table sitting around my database named #sql-49f_6.
This table was not explicitely created by me.
That said, I am using sqlalchemy to create and manage my database, so all might be due to the sqlalchemy model definition, which looks like this:
TableName = Table(<table_name>,
table_base.metadata,
Column(
<column_name_1>,
Integer(),
ForeignKey('<ref_table_name>.<ref_column_name>', onupdate='CASCADE', ondelete='CASCADE'),
nullable=False
),
Column(
<column_name_2>,
Integer(),
ForeignKey('<ref_table_name_2>.<ref_column_name_2>', onupdate='CASCADE', ondelete='CASCADE'),
nullable=False
)
)
With all of the above, can anyone suggest a solution?
Preferably without having to recreate the database; because, I need to maintain the data contained by the table.

Answer 1

I wasn't able to find the origin of the issue described above.
In need of a solution, I temporarily stored the data contained by the problematic table offline and recreated it from scratch. In this step I also added in the foreign key missing. The issue did not persist. Sorry I cannot shed light on what went wrong initially.