Customizing migrations
In some scenarios, you need to edit a migration file before you apply it. For example, to change the direction of a 1-1 relation (moving the foreign key from one side to another) without data loss, you need to move data as part of the migration - this SQL is not part of the default migration, and must be written by hand.
This guide explains how to edit migration files and gives some examples of use cases where you may want to do this.
How to edit a migration file
To edit a migration file before applying it, the general procedure is the following:
-
Make a schema change that requires custom SQL (for example, to preserve existing data)
-
Create a draft migration using:
npx prisma migrate dev --create-only
-
Modify the generated SQL file.
-
Apply the modified SQL by running:
npx prisma migrate dev
Example: Rename a field
By default, renaming a field in the schema results in a migration that will:
CREATE
a new column (for example,fullname
)DROP
the existing column (for example,name
) and the data in that column
To actually rename a field and avoid data loss when you run the migration in production, you need to modify the generated migration SQL before applying it to the database. Consider the following schema fragment - the biograpy
field is spelled wrong.
model Profile {
id Int @id @default(autoincrement())
biograpy String
userId Int @unique
user User @relation(fields: [userId], references: [id])
}
To rename the biograpy
field to biography
:
-
Rename the field in the schema:
model Profile {
id Int @id @default(autoincrement())
biograpy String
biography String
userId Int @unique
user User @relation(fields: [userId], references: [id])
} -
Run the following command to create a draft migration that you can edit before applying to the database:
npx prisma migrate dev --name rename-migration --create-only
-
Edit the draft migration as shown, changing
DROP
/DELETE
to a singleRENAME COLUMN
:- Before
- After
./prisma/migrations/20210308092620_rename_migration/migration.sqlALTER TABLE "Profile" DROP COLUMN "biograpy",
ADD COLUMN "biography" TEXT NOT NULL;./prisma/migrations/20210308092620_rename_migration/migration.sqlALTER TABLE "Profile"
RENAME COLUMN "biograpy" TO "biography"; -
Save and apply the migration:
npx prisma migrate dev
You can use the same technique to rename a model
- edit the generated SQL to rename the table rather than drop and re-create it.
Example: Use the expand and contract pattern to evolve the schema without downtime
Making schema changes to existing fields, e.g., renaming a field can lead to downtime. It happens in the time frame between applying a migration that modifies an existing field, and deploying a new version of the application code which uses the modified field.
You can prevent downtime by breaking down the steps required to alter a field into a series of discrete steps designed to introduce the change gradually. This pattern is known as the expand and contract pattern.
The pattern involves two components: your application code accessing the database and the database schema you intend to alter.
With the expand and contract pattern, renaming the field bio
to biography
would look as follows with Prisma:
-
Add the new
biography
field to your Prisma schema and create a migrationmodel Profile {
id Int @id @default(autoincrement())
bio String
biography String
userId Int @unique
user User @relation(fields: [userId], references: [id])
} -
Expand: update the application code and write to both the
bio
andbiography
fields, but continue reading from thebio
field, and deploy the code -
Create an empty migration and copy existing data from the
bio
to thebiography
fieldnpx prisma migrate dev --name copy_biography --create-only
prisma/migrations/20210420000000_copy_biography/migration.sqlUPDATE "Profile" SET biography = bio;
-
Verify the integrity of the
biography
field in the database -
Update application code to read from the new
biography
field -
Update application code to stop writing to the
bio
field -
Contract: remove the
bio
from the Prisma schema, and create a migration to remove thebio
fieldmodel Profile {
id Int @id @default(autoincrement())
bio String
biography String
userId Int @unique
user User @relation(fields: [userId], references: [id])
}npx prisma migrate dev --name remove_bio
By using this approach, you avoid potential downtime that altering existing fields that are used in the application code are prone to, and reduce the amount of coordination required between applying the migration and deploying the updated application code.
Note that this pattern is applicable in any situation involving a change to a column that has data and is in use by the application code. Examples include combining two fields into one, or transforming a 1:n
relation to a m:n
relation.
To learn more, check out the Data Guide article on the expand and contract pattern
Example: Change the direction of a 1-1 relation
To change the direction of a 1-1 relation:
-
Make the change in the schema:
model User {
id Int @id @default(autoincrement())
name String
posts Post[]
profile Profile? @relation(fields: [profileId], references: [id])
profileId Int @unique
}
model Profile {
id Int @id @default(autoincrement())
biography String
user User
} -
Run the following command to create a draft migration that you can edit before applying to the database:
npx prisma migrate dev --name rename-migration --create-only
Show CLI results⚠️ There will be data loss when applying the migration:
• The migration will add a unique constraint covering the columns `[profileId]` on the table `User`. If there are existing duplicate values, the migration will fail. -
Edit the draft migration as shown:
- Before
- After
-- DropForeignKey
ALTER TABLE "Profile" DROP CONSTRAINT "Profile_userId_fkey";
-- DropIndex
DROP INDEX "Profile_userId_unique";
-- AlterTable
ALTER TABLE "Profile" DROP COLUMN "userId";
-- AlterTable
ALTER TABLE "User" ADD COLUMN "profileId" INTEGER NOT NULL;
-- CreateIndex
CREATE UNIQUE INDEX "User_profileId_unique" ON "User"("profileId");
-- AddForeignKey
ALTER TABLE "User" ADD FOREIGN KEY ("profileId") REFERENCES "Profile"("id") ON DELETE CASCADE ON UPDATE CASCADE;
-- DropForeignKey
ALTER TABLE "Profile" DROP CONSTRAINT "Profile_userId_fkey";
-- DropIndex
DROP INDEX "Profile_userId_unique";
-- AlterTable
ALTER TABLE "User" ADD COLUMN "profileId" INTEGER;
UPDATE "User"
SET "profileId" = "Profile".id
FROM "Profile"
WHERE "User".id = "Profile"."userId";
ALTER TABLE "User" ALTER COLUMN "profileId" SET NOT NULL;
-- AlterTable
ALTER TABLE "Profile" DROP COLUMN "userId";
-- CreateIndex
CREATE UNIQUE INDEX "User_profileId_unique" ON "User"("profileId");
-- AddForeignKey
ALTER TABLE "User" ADD FOREIGN KEY ("profileId") REFERENCES "Profile"("id") ON DELETE CASCADE ON UPDATE CASCADE;
-
Save and apply the migration:
npx prisma migrate dev