Categories
IntelliJ MySQL

IntelliJ – Connect to MySQL running in Vagrant

The Database feature of IntelliJ/PHPStorm is very powerful compared to the MySQL CLI. It allows your edit your data from the UI easily without writing any MySQL commands.

Here is how you can connect your MySQL server running in a vagrant machine to the IntelliJ’s database feature and improve your productivity during development.

Create a new MySQL Data Source with these basic configuration:

Fill the basic details such as the data source name, host, port, user name, password and the database name.

Make sure you add ?useSSL=false to the JDBC URL after the form is filled. MySQL 5.7+ has SSL enabled by default.

Full JDBC url: jdbc:mysql://localhost:3306/development_database?useSSL=false

Now, with this default configuration, the connection will not work because we need to set the SSH configuration to allow IntelliJ to talk to the MySQL server running within the vagrant VM.

Goto the SSH/SSL tab, select ‘Use SSH tunnel’ and fill-in the details like this:

The value for Private key file should be the one from IdentityFile from your project vagrant directory using this command:

~/Dropbox/Projects/myproject
❯ vagrant ssh-config
Host default
  HostName 127.0.0.1
  User vagrant
  Port 2222
  UserKnownHostsFile /dev/null
  StrictHostKeyChecking no
  PasswordAuthentication no
  IdentityFile /Users/lrajasekaran/Dropbox/Projects/myproject/.vagrant/machines/default/virtualbox/private_key/private_key
  IdentitiesOnly yes
  LogLevel FATAL

Now click ‘Test Connection’ from this dialog to ensure the SSH connection is working as expected. Once the SSH connection is setup, close this dialog, go to ‘General’ tab and click ‘Test Connection’ to test the actual database connection using the SSH connection made previously. Once this successful, goto ‘Schemas’ tab and select your database name and click ‘Apply’.

Now you can view all your tables from the Database tool window on the right. Right click on any table and click ‘Jump to Editor’ to see the table rows:

Any value can be edited directly from this UI and then saved to the DB by right clicking the row and clicking ‘Submit’

Categories
Database MySQL

5 Reasons MySQL Foreign Key constraints fail to create

Finding out why Foreign key creation fail

When MySQL is unable to create a Foreign Key, it throws out this generic error message:

ERROR 1215 (HY000): Cannot add foreign key constraint

– The most useful error message ever.

Fortunately, MySQL has this useful command that can give the actual reason about why it could not create the Foreign Key.

mysql> SHOW ENGINE INNODB STATUS;

That will print out lots of output but the part we are interested in is under the heading ‘LATEST FOREIGN KEY ERROR’:

------------------------
LATEST FOREIGN KEY ERROR
------------------------
2020-08-29 13:40:56 0x7f3cb452e700 Error in foreign key constraint of table test_database/my_table:
there is no index in referenced table which would contain
the columns as the first columns, or the data types in the
referenced table do not match the ones in table. Constraint:
,
CONSTRAINT idx_name FOREIGN KEY (employee_id) REFERENCES employees (id)
The index in the foreign key in table is idx_name
Please refer to http://dev.mysql.com/doc/refman/5.7/en/innodb-foreign-key-constraints.html for correct foreign key definition.

This output could give you some clue about the actual reason why MySQL could not create your Foreign Key

Reason #1 – Missing unique index on the referenced table

This is probably the most common reason why MySQL won’t create your Foreign Key constraint. Let’s look at an example with a new database and new tables:

In the all below examples, we’ll use a simple ‘Employee to Department” relationship:

mysql> CREATE DATABASE foreign_key_1;
Query OK, 1 row affected (0.00 sec)
mysql> USE foreign_key_1;
Database changed

mysql> CREATE TABLE employees(
    ->     id int,
    ->     name varchar(20),
    ->     department_id int
    -> );
Query OK, 0 rows affected (0.08 sec)

mysql> CREATE TABLE departments(
    ->     id int,
    ->     name varchar(20)
    -> );
Query OK, 0 rows affected (0.07 sec)

As you may have noticed, we have not created the table with PRIMARY KEY or unique indexes. Now let’s try to create Foreign Key constraint between employees.department_id column and departments.id column:

mysql> ALTER TABLE employees ADD CONSTRAINT fk_department_id FOREIGN KEY idx_employees_department_id (department_id) REFERENCES departments(id);
ERROR 1215 (HY000): Cannot add foreign key constraint

Let’s look at the detailed error:

mysql> SHOW ENGINE INNODB STATUS;
------------------------
LATEST FOREIGN KEY ERROR
------------------------
2020-08-31 09:25:13 0x7fddc805f700 Error in foreign key constraint of table foreign_key_1/#sql-5ed_49b:
FOREIGN KEY idx_employees_department_id (department_id) REFERENCES departments(id):
Cannot find an index in the referenced table where the
referenced columns appear as the first columns, or column types
in the table and the referenced table do not match for constraint.
Note that the internal storage type of ENUM and SET changed in
tables created with >= InnoDB-4.1.12, and such columns in old tables
cannot be referenced by such columns in new tables.
Please refer to http://dev.mysql.com/doc/refman/5.7/en/innodb-foreign-key-constraints.html for correct foreign key definition.

This is because we don’t have any unique index on the referenced table i.e. departments. We have two ways of fixing this:

Option 1: Primary Keys

Let’s fix this by adding a primary key departments.id

mysql> ALTER TABLE departments ADD PRIMARY KEY (id);
Query OK, 0 rows affected (0.20 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> ALTER TABLE employees ADD CONSTRAINT fk_department_id FOREIGN KEY idx_employees_department_id (department_id) REFERENCES departments(id);
Query OK, 0 rows affected (0.19 sec)
Records: 0  Duplicates: 0  Warnings: 0

Option 2: Unique Index

mysql> CREATE UNIQUE INDEX idx_department_id ON departments(id);
Query OK, 0 rows affected (0.13 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> ALTER TABLE employees ADD CONSTRAINT fk_department_id FOREIGN KEY idx_employees_department_id (department_id) REFERENCES departments(id);
Query OK, 0 rows affected (0.21 sec)
Records: 0  Duplicates: 0  Warnings: 0

Reason #2 – Different data types on the columns

MySQL requires the columns involved in the foreign key to be of the same data types.

mysql> CREATE DATABASE foreign_key_1;
Query OK, 1 row affected (0.00 sec)

mysql> USE foreign_key_1;
Database changed

mysql> CREATE TABLE employees(
    ->     id int,
    ->     name varchar(20),
    ->     department_id int,
    ->     PRIMARY KEY (id)
    -> );
Query OK, 0 rows affected (0.06 sec)

mysql> CREATE TABLE departments(
    ->     id char(20),
    ->     name varchar(20),
    ->     PRIMARY KEY (id)
    -> );
Query OK, 0 rows affected (0.07 sec)

You may have noticed that employees.department_id is int while departments.id is char(20). Let’s try to create a foreign key now:

mysql> ALTER TABLE employees ADD CONSTRAINT fk_department_id FOREIGN KEY idx_employees_department_id (department_id) REFERENCES departments(id);
ERROR 1215 (HY000): Cannot add foreign key constraint

Let’s fix the type of departments.id and try to create the foreign key again:

mysql> ALTER TABLE departments MODIFY id INT;
Query OK, 0 rows affected (0.18 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> ALTER TABLE employees ADD CONSTRAINT fk_department_id FOREIGN KEY idx_employees_department_id (department_id) REFERENCES departments(id);
Query OK, 0 rows affected (0.26 sec)
Records: 0  Duplicates: 0  Warnings: 0

It works now!

Reason #3 – Different collation/charset type on the table

This is a surprising reason and hard to find out. Let’s create two tables with different collation (or also called charset):

Let’s start from scratch to explain this scenario:

mysql> CREATE DATABASE foreign_key_1;                                                                                        Query OK, 1 row affected (0.00 sec)

mysql> USE foreign_key_1;                                                                                                    Database changed

mysql> CREATE TABLE employees(
    ->     id int,
    ->     name varchar(20),
    ->     department_id int,
    ->     PRIMARY KEY (id)
    -> ) ENGINE=InnoDB CHARACTER SET=utf8;
Query OK, 0 rows affected (0.06 sec)

mysql> CREATE TABLE departments(
    ->     id int,
    ->     name varchar(20),
    ->     PRIMARY KEY (id)
    -> ) ENGINE=InnoDB CHARACTER SET=latin1;
Query OK, 0 rows affected (0.08 sec)

You may notice that we are using a different character set (utf8 and latin1` for both these tables. Let’s try to create the foreign key:

mysql> ALTER TABLE employees ADD CONSTRAINT fk_department_id FOREIGN KEY idx_employees_department_id (department_id) REFERENCES departments(id);
ERROR 1215 (HY000): Cannot add foreign key constraint

It failed because of different character sets. Let’s fix that.

mysql> SET foreign_key_checks = 0; ALTER TABLE departments CONVERT TO CHARACTER SET utf8 COLLATE utf8_general_ci; SET foreign_key_checks = 1;
Query OK, 0 rows affected (0.00 sec)

Query OK, 0 rows affected (0.18 sec)
Records: 0  Duplicates: 0  Warnings: 0

Query OK, 0 rows affected (0.00 sec)

mysql> ALTER TABLE employees ADD CONSTRAINT fk_department_id FOREIGN KEY idx_employees_department_id (department_id) REFERENCES departments(id);
Query OK, 0 rows affected (0.20 sec)
Records: 0  Duplicates: 0  Warnings: 0

If you have many tables with a different collation/character set, use this script to generate a list of commands to fix all tables at once:

mysql --database=your_database -B -N -e "SHOW TABLES" | awk '{print "SET foreign_key_checks = 0; ALTER TABLE", $1, "CONVERT TO CHARACTER SET utf8 COLLATE utf8_general_ci; SET foreign_key_checks = 1; "}'

Reason #4 – Different collation types on the columns

This is a rare reason, similar to reason #3 above but at a column level.

Let’s try to reproduce this from scratch:

mysql> CREATE DATABASE foreign_key_1;                                                                                        Query OK, 1 row affected (0.00 sec)

mysql> USE foreign_key_1;                                                                                                    Database changed

mysql> CREATE TABLE employees(
    ->     id int,
    ->     name varchar(20),
    ->     department_id char(26) CHARACTER SET utf8,
    ->     PRIMARY KEY (id)
    -> );
Query OK, 0 rows affected (0.07 sec)

mysql> CREATE TABLE departments(
    ->     id char(26) CHARACTER SET latin1,
    ->     name varchar(20),
    ->     PRIMARY KEY (id)
    -> );
Query OK, 0 rows affected (0.08 sec)

We are using a different character set for employees.department_id and departments.id (utf8 and latin1). Let’s check if the Foreign Key can be created:

mysql> ALTER TABLE employees ADD CONSTRAINT fk_department_id FOREIGN KEY idx_employees_department_id (department_id) REFERENCES departments(id);
ERROR 1215 (HY000): Cannot add foreign key constraint

Nope, as expected. Let’s fix that by changing the character set of departments.id to match with employees.department_id:

mysql> ALTER TABLE departments MODIFY id CHAR(26) CHARACTER SET utf8;
Query OK, 0 rows affected (0.20 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> ALTER TABLE employees ADD CONSTRAINT fk_department_id FOREIGN KEY idx_employees_department_id (department_id) REFERENCES departments(id);
Query OK, 0 rows affected (0.20 sec)
Records: 0  Duplicates: 0  Warnings: 0

It works now!

Reason #5 -Inconsistent data

This would be the most obvious reason. A foreign key is to ensure that your data remains consistent between the parent and the child table. So when you are creating the foreign key, the existing data is expected to be already consistent.

Let’s setup some inconsistent data to reproduce this problem:

mysql> CREATE DATABASE foreign_key_1;                                                                                        Query OK, 1 row affected (0.00 sec)

mysql> USE foreign_key_1;                                                                                                    Database changed

mysql> CREATE TABLE employees(
    ->     id int,
    ->     name varchar(20),
    ->     department_id int,
    ->     PRIMARY KEY (id)
    -> );
Query OK, 0 rows affected (0.06 sec)

mysql> CREATE TABLE departments(
    ->     id int,
    ->     name varchar(20),
    ->     PRIMARY KEY (id)
    -> );
Query OK, 0 rows affected (0.08 sec)

Let’s insert a department_id in employees table that will not exist in departments.id:

mysql> INSERT INTO employees VALUES (1, 'Amber', 145);
Query OK, 1 row affected (0.01 sec)

Let’s create a foreign key now and see if it works:

mysql> ALTER TABLE employees ADD CONSTRAINT fk_department_id FOREIGN KEY idx_employees_department_id (department_id) REFERENCES departments(id);

ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`foreign_key_1`.`#sql-5ed_49b`, CONSTRAINT `fk_department_id` FOREIGN KEY (`department_id`) REFERENCES `departments` (`id`))

This error message is atleast more useful. We can fix this in two ways. Either by adding the missing department in departments table or by deleting all the employees with the missing department. We’ll do the first option now:

mysql> INSERT INTO departments VALUES (145, 'HR');
Query OK, 1 row affected (0.00 sec)

Let’s try to create the Foreign Key again:

mysql> ALTER TABLE employees ADD CONSTRAINT fk_department_id FOREIGN KEY idx_employees_department_id (department_id) REFERENCES departments(id);
Query OK, 1 row affected (0.24 sec)
Records: 1  Duplicates: 0  Warnings: 0

It worked this time.

So we have seen 5 different ways a Foreign Key creation can fail and possible solutions of how we can fix them. If you have encountered a reason not listed above, add them in the comments.