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Description

This error indicates that a foreign key constraint has been violated, preventing the operation from completing. It commonly occurs when attempting to insert or update data into a child table with a foreign key value that does not exist in the referenced parent table, or during table creation if the foreign key definition refers to non-existent or incompatible columns.

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Error Message

invalid foreign key

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Known Causes

3 known causes

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Referenced Parent Key Missing

The value being inserted or updated into the foreign key column does not exist in the primary or unique key column of the referenced parent table.

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Data Type Mismatch

The data types of the foreign key column in the child table and the referenced key column in the parent table are not compatible.

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Non-Existent Referenced Object

The foreign key constraint attempts to reference a table or column that does not exist in the database schema.

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Solutions

3 solutions available

1. Verify Parent Table and Column Existence easy

Ensure the referenced table and its primary key column exist and are spelled correctly.

1

Connect to your PostgreSQL database using `psql` or your preferred SQL client.

psql -U your_username -d your_database

2

Check if the referenced table exists in the database.

SELECT tablename FROM pg_tables WHERE schemaname = 'public' AND tablename = 'your_parent_table_name';

3

If the table exists, check if the referenced column is the primary key or has a unique constraint.

SELECT a.attname FROM   pg_index i JOIN   pg_attribute a ON a.attrelid = i.indrelid AND a.attnum = ANY(i.indkey) WHERE  i.indrelid = 'your_parent_table_name'::regclass AND i.indisprimary;

4

If the column is not the primary key, check for a unique constraint on it.

SELECT conname FROM pg_constraint WHERE conrelid = 'your_parent_table_name'::regclass AND contype = 'u' AND 'your_parent_column_name' = ANY(conkey::int[]);

5

If the table or column is missing or misspelled, correct the foreign key definition in your `ALTER TABLE` statement.

ALTER TABLE your_child_table_name ADD CONSTRAINT your_fk_constraint_name FOREIGN KEY (your_child_column_name) REFERENCES your_parent_table_name (your_parent_column_name);

2. Match Data Types of Referenced Columns medium

Ensure the data types of the foreign key column and the referenced primary/unique key column are identical.

1

Identify the data types of the foreign key column in the child table and the referenced column in the parent table.

SELECT a.attname, format_type(a.atttypid, a.atttypmod) AS data_type FROM pg_attribute a WHERE a.attrelid = 'your_child_table_name'::regclass AND a.attname = 'your_child_column_name';

SELECT a.attname, format_type(a.atttypid, a.atttypmod) AS data_type FROM pg_attribute a WHERE a.attrelid = 'your_parent_table_name'::regclass AND a.attname = 'your_parent_column_name';

2

If the data types do not match, you will need to alter one of the tables to make them compatible. This might involve adding a new column with the correct type, migrating data, and then dropping the old column. Be cautious with this operation as it can be complex.

-- Example: If child column is TEXT and parent is INT
-- First, add a new integer column to the child table
ALTER TABLE your_child_table_name ADD COLUMN your_child_column_name_new INTEGER;

-- Then, migrate data (handle potential conversion errors)
UPDATE your_child_table_name SET your_child_column_name_new = CAST(your_child_column_name AS INTEGER);

-- Once data is migrated and validated, you can alter the table to drop the old column and rename the new one, or directly add the FK to the new column.

3

After ensuring data types match, re-attempt to add the foreign key constraint.

ALTER TABLE your_child_table_name ADD CONSTRAINT your_fk_constraint_name FOREIGN KEY (your_child_column_name) REFERENCES your_parent_table_name (your_parent_column_name);

3. Address Existing Data Violations medium

Remove or update rows in the child table that violate the foreign key constraint before it can be added.

1

Identify rows in the child table where the foreign key column's value does not exist in the parent table's referenced column.

SELECT c.* FROM your_child_table_name c LEFT JOIN your_parent_table_name p ON c.your_child_column_name = p.your_parent_column_name WHERE p.your_parent_column_name IS NULL AND c.your_child_column_name IS NOT NULL;

2

Decide how to handle these violating rows. Options include:

3

Delete the violating rows (use with extreme caution).

DELETE FROM your_child_table_name WHERE your_child_column_name NOT IN (SELECT your_parent_column_name FROM your_parent_table_name);

4

Update the violating rows to reference existing parent records, or set them to NULL if the foreign key allows it.

-- Example: Update to a valid parent ID
UPDATE your_child_table_name SET your_child_column_name = (SELECT your_parent_column_name FROM your_parent_table_name LIMIT 1) WHERE your_child_column_name NOT IN (SELECT your_parent_column_name FROM your_parent_table_name);

-- Example: Set to NULL (if the column is nullable)
-- ALTER TABLE your_child_table_name ALTER COLUMN your_child_column_name DROP NOT NULL; -- if it was NOT NULL
-- UPDATE your_child_table_name SET your_child_column_name = NULL WHERE your_child_column_name NOT IN (SELECT your_parent_column_name FROM your_parent_table_name);

5

After cleaning up the data, re-attempt to add the foreign key constraint.

ALTER TABLE your_child_table_name ADD CONSTRAINT your_fk_constraint_name FOREIGN KEY (your_child_column_name) REFERENCES your_parent_table_name (your_parent_column_name);

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Related Errors

5 related errors

PostgreSQL Error 42830: Invalid Foreign Key Constraint