An issue to note when modifying fields in Django

Problem: The Django 1.7.8 table Users and Tenant do many-to-many associations. Django automatically creates a table users_tenants (corresponding to user_id and tenant_id) class users (models. Model): Tenants = models. Manytomanyfield (Tenant) when executing user.tenants.add (Tenant), reported the following issue: ERROR: (1452, ' cannot add or update a child row:a foreign K EY constraint fails (' iaasms '. ' users_tenants ', constraint ' users_tenants_users_id_3d9ac202e9485f18_fk_users_user_id ' FOREIGN key (' users_id ') REFERENCES ' users ' (' user_id ')) A: Although the foreign key check is turned off in the default in setting-DATABASES: ' OPTIONS ': {' Init_command ': ' SET foreign_key_checks = 0; ',} This error can be eliminated, but the result of the associated query User.tenants.all () is empty.
Root cause: After the first migrate, the length of the Users_tenants field tenant_id is derived from the length of tenant_id in table tenant. However, I later increased the length of the tenant_id in tenant, and after migrate, the length of tenant_id in users_tenants did not change. Therefore, the length of the tenant_id of users_tenants is not sufficient in the execution of User.tenants.add (tenant).
When the foreign key check is turned off in the default in setting-DATABASES, the result of the associated query is empty, although no error is made, but the value of Users_tenants's tenant_id is not complete (the actual data is greater than the length of the word).
WORKAROUND: Delete the migration files and databases and recreate the database.