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Mechanism for adding multiple foreign key constraints at once #31

simonw opened this issue Jun 27, 2019 · 0 comments

Mechanism for adding multiple foreign key constraints at once #31

simonw opened this issue Jun 27, 2019 · 0 comments


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simonw commented Jun 27, 2019

Needed by db-to-sqlite. It currently works by collecting all of the foreign key relationships it can find and then applying them at the end of the process.

The problem is, the add_foreign_key() method looks like this:

with self.db.conn:
cursor = self.db.conn.cursor()
schema_version = cursor.execute("PRAGMA schema_version").fetchone()[0]
cursor.execute("PRAGMA writable_schema = 1")
old_sql = self.schema
extra_sql = ",\n FOREIGN KEY({column}) REFERENCES {other_table}({other_column})\n".format(
column=column, other_table=other_table, other_column=other_column
# Stick that bit in at the very end just before the closing ')'
last_paren = old_sql.rindex(")")
new_sql = old_sql[:last_paren].strip() + extra_sql + old_sql[last_paren:]
"UPDATE sqlite_master SET sql = ? WHERE name = ?", (new_sql,
cursor.execute("PRAGMA schema_version = %d" % (schema_version + 1))
cursor.execute("PRAGMA writable_schema = 0")
# Have to VACUUM outside the transaction to ensure .foreign_keys property
# can see the newly created foreign key.

That means it's doing a full VACUUM for every single relationship it sets up - and if you have hundreds of foreign key relationships in your database this can take hours.

I think the right solution is to have a .add_foreign_keys(list_of_args) method which does the bulk operation and then a single VACUUM. .add_foreign_key(...) can then call the bulk action with a single list item.

simonw added a commit that referenced this issue Jun 28, 2019
Still needs unit tests and documentation.
@simonw simonw mentioned this issue Jun 28, 2019
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@simonw simonw closed this as completed in 997d875 Jun 29, 2019
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