MySQL Foreign Key Error 1005 errno 150 primary key as foreign key

When creating a foreign key constraint, MySQL requires a usable index on both the referencing table and also on the referenced table. The index on the referencing table is created automatically if one doesn't exist, but the one on the referenced table needs to be created manually (Source). Yours appears to be missing.

Test case:

CREATE TABLE tbl_a (
    id int PRIMARY KEY,
    some_other_id int,
    value int
) ENGINE=INNODB;
Query OK, 0 rows affected (0.10 sec)

CREATE TABLE tbl_b (
    id int PRIMARY KEY,
    a_id int,
    FOREIGN KEY (a_id) REFERENCES tbl_a (some_other_id)
) ENGINE=INNODB;
ERROR 1005 (HY000): Can't create table 'e.tbl_b' (errno: 150)

But if we add an index on some_other_id:

CREATE INDEX ix_some_id ON tbl_a (some_other_id);
Query OK, 0 rows affected (0.11 sec)
Records: 0  Duplicates: 0  Warnings: 0

CREATE TABLE tbl_b (
    id int PRIMARY KEY,
    a_id int,
    FOREIGN KEY (a_id) REFERENCES tbl_a (some_other_id)
) ENGINE=INNODB;
Query OK, 0 rows affected (0.06 sec)

This is often not an issue in most situations, since the referenced field is often the primary key of the referenced table, and the primary key is indexed automatically.

Double check that the foreign keys have exactly the same type as the field you've got in this table. For example, both should be Integer(10), or Varchar (8), even the number of characters.

I realize this is an old post, but it ranks high in Google, so I'm adding what I figured out for MY problem. If you have a mix of table types (e.g. MyISAM and InnoDB), you will get this error as well. In this case, InnoDB is the default table type, but one table needed fulltext searching so it was migrated to MyISAM. In this situation, you cannot create a foreign key in the InnoDB table that references the MyISAM table.