04. Testing and Data Seeding

23. Transactions

A transaction means:

• Run multiple database operations together.
• If one fails, all fail.
• If all pass, all save.

Example:

await client.$transaction([
  client.user.create({
    data: {
      username: "transaction_user",
      password: "secret",
      age: 30,
      city: "mumbai",
    },
  }),
  client.todo.create({
    data: {
      title: "Transaction todo",
      description: "Created inside transaction",
      done: false,
      userId: 1,
    },
  }),
]);

Important: the array form is good when the operations do not need data from each other. In the example above, userId: 1 only works if a user with id 1 already exists. The todo cannot use the id from the user created in the previous item of the same array.

Interactive transaction:

await client.$transaction(async (tx) => {
  const user = await tx.user.create({
    data: {
      username: "interactive_user",
      password: "secret",
      age: 28,
      city: "pune",
    },
  });

  await tx.todo.create({
    data: {
      title: "Todo for interactive user",
      description: "Created after user",
      done: false,
      userId: user.id,
    },
  });
});

Use the interactive transaction when one query depends on the result of another query. Here the todo uses user.id, so it belongs to the exact user that was just created.

24. Migrations

Migration means:

• You changed schema.prisma.
• Prisma creates SQL to update your database.
• The SQL is stored in prisma/migrations.

Development Workflow: Change Your Database Safely

Use this workflow whenever you add, remove, or change a model, field, relation, index, or constraint.

1. Change prisma/schema.prisma.
2. Format and check the schema:

npx prisma format

3. Create and apply a development migration with a clear name:

npx prisma migrate dev --name add_todo_priority

4. Regenerate Prisma Client after schema changes:

npx prisma generate

5. Test your code, then commit both prisma/schema.prisma and the new folder inside prisma/migrations/.

migrate dev creates SQL migration files and applies them to your development database. A meaningful migration name makes the history easy to understand later.

Commands and When To Use Them

Command	Use it for	Important note
npx prisma migrate dev --name name_here	Creating and applying a new migration on your own development database	Use after changing the schema.
npx prisma generate	Updating the TypeScript client types and methods	Run after schema or generator changes.
npx prisma migrate status	Checking whether migrations are applied and whether Prisma sees a problem	Read-only check; safe before deploying.
npx prisma migrate deploy	Applying existing committed migrations to staging or production	Does not create a new migration.
npx prisma migrate reset	Starting over on a disposable development database	Deletes all database data. Never use it in production.
npx prisma db push	Fast prototyping when migration history does not matter	Does not create migration files; do not use it for a shared or production database.

Working With Existing Migrations

• Do not edit or delete a migration that has already been applied to a shared, staging, or production database.
• If you need to correct an earlier change, change schema.prisma and create a new migration.
• When you pull a teammate's or another branch's migration files, run npx prisma migrate dev on your development database, then run npx prisma generate.
• Before deploying, commit the schema and migration folder together. Your database change is incomplete if one is missing.
• Use npx prisma migrate deploy in staging or production. Do not use migrate dev there.

Reset Database and Apply Migrations Again

npx prisma migrate reset

Use this carefully because it deletes existing data.

For this learning project, prefer migrate dev over db push, because migration files teach you exactly how the schema becomes SQL.

25. Prisma Studio

Open a browser UI for your database:

npx prisma studio

Use Prisma Studio to:

• See users
• See todos
• Add rows manually
• Edit rows
• Understand relations visually

26. Seeding

Seeding means adding sample data automatically.

Example idea:

await client.user.create({
  data: {
    username: "seed_user",
    password: "secret",
    age: 20,
    city: "lucknow",
    todos: {
      create: {
        title: "Seeded todo",
        description: "Created from seed script",
        done: false,
      },
    },
  },
});

For this project, a real seed file could be:

// prisma/seed.ts
import { PrismaClient } from "@prisma/client";

const client = new PrismaClient();

async function main() {
  await client.user.upsert({
    where: {
      username: "seed_user",
    },
    update: {},
    create: {
      username: "seed_user",
      password: "secret",
      age: 20,
      city: "lucknow",
      todos: {
        create: {
          title: "Seeded todo",
          description: "Created from seed script",
          done: false,
        },
      },
    },
  });
}

main()
  .catch((error) => {
    console.error(error);
    process.exit(1);
  })
  .finally(async () => {
    await client.$disconnect();
  });

Then configure Prisma to run that file.

Your project already has prisma.config.ts, so add a seed command inside migrations:

import { defineConfig, env } from "prisma/config";

export default defineConfig({
  schema: "prisma/schema.prisma",
  migrations: {
    path: "prisma/migrations",
    seed: "tsx prisma/seed.ts",
  },
  datasource: {
    url: env("DATABASE_URL"),
  },
});

Because this project does not currently have tsx installed, install it first:

npm install -D tsx

Run the seed:

npx prisma db seed

npx prisma migrate reset can also run the seed after resetting the database. Use reset carefully because it deletes data.

27. Enums

Enums are fixed allowed values.

Example:

enum TodoStatus {
  PENDING
  DONE
  CANCELLED
}

model Todo {
  id          Int        @id @default(autoincrement())
  title       String
  description String?
  status      TodoStatus @default(PENDING)
}

Use enum when a field should only allow specific values.

28. Raw SQL

Prisma lets you run raw SQL when needed.

const result = await client.$queryRaw`SELECT * FROM "User"`;
console.log(result);

Use Prisma Client methods first. Use raw SQL only when Prisma Client cannot easily express the query.

29. Error Handling

Duplicate username can throw an error because username is unique.

Simple error handling:

try {
  await client.user.create({
    data: {
      username: "utsav",
      password: "secret",
      age: 24,
      city: "lucknow",
    },
  });
} catch (error) {
  console.error("Something went wrong:", error);
}

For practice, use upsert to avoid duplicate create errors.

30. Best Learning Order

Follow this order:

1. Schema models
2. Migrations
3. Prisma Client setup
4. create
5. findMany
6. findUnique
7. findFirst
8. Filtering
9. update
10. delete
11. Relations
12. include
13. select
14. Nested writes
15. Ordering
16. Pagination
17. Counting
18. Transactions
19. Indexes and constraints
20. Enums
21. Seeding
22. Prisma Studio