If you shuddered in recognition when reading that, then you know what the safe way to do this is: create the column without foreignkey: true; then use PostgreSQL's ALTER TABLE ADD CONSTRAINT with the NOT VALID flag, which will update the catalogs but not actually attempt to verify that the data in the column are currently valid; then, finally. To add foreign key to an existing reference, create a new migration to add a foreign key: class AddForeignKeyToUploads generate a migration using the following command.

• Rails Generate Migration Add Column Foreign Key In Word
• Rails Generate Migration Add Column
• Rails Generate Migration Add Foreign Key
• Add Numbers

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There are many ways to add the foreign key column that's required by our has_many and belongs_to associations, and I want to take a moment to show you a couple more of them.

There are many ways to add the foreign key column that's required by our has_many and belongs_to associations, and I want to take a moment to show you a couple more of them.

• If you want, you can use the references column type in migrations instead.
  • bin/rails generate migration AddPostToComments post:references
  • That will create a migration with a call to the add_reference method instead of add_column. add_reference takes a symbol with a table name, and a symbol with the name of a model to add a foreign key for. It'll create a column whose name begins with that model name, and ends in _id. And since it's always desirable to have an index on foreign key columns, add_reference will add an index as well. So add_reference :comments, :post will create a post_id column in the comments table just like add_column did, but it will also add an index on that column automatically.
  • The foreign_key: true argument will set up a foreign key constraint on databases that support it.

Rails Generate Migration Add Column Foreign Key In Word

Without a foreign key constraint, we could create a comment with a post_id field set to 999, even if there was no record in the post table with an id of 999. With a foreign key constraint, the database would prevent such a record from even being saved. Foreign key constraints help keep bad data from sneaking into your database.

Note that the adapter for the SQLite database that Rails uses by default doesn't support foreign key constraints. Your migration will still work, and it's a good idea to get in the habit of adding the constraints in your migrations. But if you want the database to actually enforce the constraints, you'll need to switch to another database like MySQL or PostgreSQL.

Rails Generate Migration Add Column

If we know we're going to need an association when we're first creating a model, we can set the necessary columns up then, too.

• Let's re-generate the Comment model. We can replace the two migrations with a single migration that creates the comments table and adds a post_id column along with the other columns: rails g model Comment content:text name:string post:references.
  • We can allow it to overwrite the existing model class file, as well as another file related to tests.
  • Don't forget to run bin/rails db:migrate.
  • We should now be able to create comments associated with a post again.

Rails Generate Migration Add Foreign Key

• 0:00

  There are many ways to add the foreign_keycolumn that's required by our has_many and

• 0:04

  belongs_to associations.

• 0:06

  And I wanna take a moment toshow you a couple more of them.

• 0:09

  Let's undo our changes to the databaseby rolling back the latest migration.

• 0:13

  We'll run bin/rails.

• 0:16

  And we'll run the db:rollback task.

• 0:20

  That'll go through the commands in thelatest migration and undo them one by one.

• 0:25

  Then let's get rid ofthe migration we created.

• 0:27

  We're gonna do thatwith bin/rails destroy,

• 0:31

  which you can consider to bethe opposite of the generate command.

• 0:35

  We're gonna destroy the migrationnamed AddPostToComments.

• 0:44

  That's the latest migration we created.

• 0:47

  That'll go through,

• 0:48

  find all the files that were createdby the generator, and delete them.

• 0:52

  Now let's redo our previous migrationusing the references column type instead.

• 0:56

  So we're gonna run bin/rails generate.

• 1:02

  We're gonna generate a migration.

• 1:04

  And again,it'll be named AddPostToComments.

• 1:09

  This time, instead of a type of integer,we're going to give it

• 1:14

  a column name of post anda type of references.

• 1:21

  That'll create a new migration file.

• 1:27

  And the file will have a call tothe add_reference method instead of

• 1:31

  add_column.

• 1:32

  add_reference takes a symbol with a tablename and a symbol with the name of warcraft 3 frozen throne game

• 1:36

  a model to add a foreign_key for,in this case, the post model.

• 1:41

  It'll create a column whose namebegins with that model name and

• 1:45

  ends in underscore ID.

• 1:47

  And since it's always desirable tohave an index on foreign_key columns,

• 1:50

  add_reference will add an index as well.

• 1:53

  So add_reference :comments,

• 1:54

  :post will create a post_id column in thecomments table, just like add_column did.

• 1:59

  But it'll also add an indexon that column automatically.

• 2:03

  The foreign_key: true argument willset up a foreign_key constraint

• 2:07

  on databases that support it.

• 2:10

  Without a foreign_key constraint, we couldcreate a comment with a post_id field set

• 2:14

  to 999, even if there was no recordin the post table with an ID of 999.

• 2:21

  But with a foreign_key constraint,

• 2:23

  the database would prevent sucha record from even being saved.

• 2:27

  Note that the adapter for the SQL-likedatabase that Rails uses by default

• 2:31

  doesn't support foreign_key constraints.

• 2:34

  Your migration will still work.

• 2:35

  And it's a good idea to get in the habitof adding the constraints in your

• 2:38

  migrations.

• 2:40

  But if you want the database toactually enforce the constraints,

• 2:43

  you'll need to switch to anotherdatabase like MySQL or PostgreSQL.

• 2:47

  See the teacher's notes if you'dlike more info on doing so.

• 2:51

  Now that we have our new migration,let's try running it.

• 2:54

  bin/rails db:migrate.

• 3:01

  It'll add a post_id field backto our comments table again.

• 3:04

  And if we launch a Rails console, we'll beable to add comments to our post again.

• 3:11

  So if we were to take the first post and

• 3:16

  create a comment on itwith comments.create,

• 3:21

  we'll give it content of HI.

• 3:27

  And the commenter name is gonna be Jay.

• 3:31

  And if we pretty print the listof the first post comments,

• 3:37

  we can see the comment's been added.

• 3:42

  If we know we're going to needan association when we're first

• 3:45

  creating a model, we can setthe necessary columns up then, too.

• 3:49

  Let's start by getting our databaseback to the point it was at

• 3:52

  before we created the comments model.

• 3:54

  Since we've only set up the commentstable on our development machine,

• 3:57

  we can just undo the migrationsthat created it.

• 4:00

  We can list out the migrations in ourdb/migrate directory with ls db/migrate.

• 4:08

  You might need to use a slightly differentcommand to list the directory contents if

• 4:12

  you're on Windows.

• 4:13

  We can see that the two most recentmigrations are add_post_to_comments and

• 4:18

  create_comments.

• 4:20

  Let's roll back our migration that addsthe post_id field to the comments table.

• 4:24

  bin/rails db:rollback.

• 4:30

  Then let's roll back the migrationthat created the comments table.

• 4:33

  bin/rails db:rollback again.

• 4:36

  Now let's get rid of thoselast two migrations.

• 4:39

  bin/rails destroy,

• 4:44

  The migration named AddPostToComments.

• 4:52

  Battlefield 4 premium key generator. And then we'll also destroythe migration named CreateComments.

• 5:00

  Now let's regenerate the comment model.

• 5:02

  We can replace the two migrations with asingle migration that creates the comments

• 5:07

  table and adds a post_id columnalong with the other columns.

• 5:10

  So we'll say bin/rails

• 5:14

  generate model Comment.

• 5:19

  We'll set up a contentattribute of type text and

• 5:24

  commenter name attribute of type string.

• 5:28

  And post:references willset up the post_id field.

• 5:35

  If it asks to overwrite any existingfiles, it should be okay to allow it.

• 5:39

  We generated the migration, sodon't forget to run bin/rails db:migrate.

• 5:47

  And if we launch bin/rails console,

• 5:52

  we should be able to create commentsassociated with post again.

• 5:55

  So let's say post = Post.first, Rsa key pair generator in java.

• 6:01

  post.comments.create.

• 6:05

  And we'll give it content of Hi anda commenter name of Alena.

• 6:15

  And if we run Post.first.comments,it should bring up our new comment.

• 6:22

  Now you know several ways tosetup a has_many association.

• 6:26

  In the next few videos, we're goingto take a look at some of the other

• 6:28

  associations that Active Record supports

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