Foreign key error when creating table (errno 150) - mysql

Question

I'm trying to create a relationship between two tables using the following queries:
create table card
(id int not null auto_increment,
post_id bigint(20) unsigned not null,
primary key(id));
create table user_comment
(id int not null auto_increment,
comment_author longtext,
comment_post_id bigint(20) unsigned not null,
primary key (id),
foreign key (comment_post_id) references card(post_id));
However it gives me the following error message:
ERROR 1005 (HY000) at line 9: Can't create table 'test.user_comment'
(errno: 150)
If I execute the command for showing innodb status:
show engine innodb status;
It shows this message:
LATEST FOREIGN KEY ERROR
Error in foreign key constraint of table test/user_comment: foreign key
(comment_post_id) references card(post_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.
But I still can't figure out how to fix the problem.
Any ideas?

Answer 1

From the docs:
MySQL requires indexes on foreign keys and referenced keys so that foreign key checks can be fast and not require a table scan. In the referencing table, there must be an index where the foreign key columns are listed as the first columns in the same order. Such an index is created on the referencing table automatically if it does not exist. This index might be silently dropped later, if you create another index that can be used to enforce the foreign key constraint. index_name, if given, is used as described previously.
Create an index on card (post_id).

Answer 2

Just use this command once before creating the table:
SET FOREIGN_KEY_CHECKS=0;