api-database-knex

Knex.js Patterns

Quick Guide: Use Knex.js (v3.x) as a SQL query builder for PostgreSQL, MySQL, SQLite, and MSSQL. Initialize the knex instance once per application (it creates a connection pool internally via tarn.js). Set pool min: 0 so idle connections are released. Always use parameterized bindings (? for values, ?? for identifiers) in knex.raw() -- never interpolate user input. Wrap multi-table writes in knex.transaction() and always return or await the promise (otherwise the transaction hangs). Use .returning() on PostgreSQL/MSSQL for inserted/updated rows -- it is a no-op on MySQL/SQLite. Call knex.destroy() on graceful shutdown to drain the pool.

---

<critical_requirements>

CRITICAL: Before Using This Skill

All code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering, import type, named constants)

(You MUST initialize the knex instance ONCE per application and reuse it -- creating multiple instances leaks connection pools)

(You MUST use parameterized bindings (? for values, ?? for identifiers) in ALL knex.raw() calls -- string interpolation causes SQL injection)

(You MUST return or await the promise inside knex.transaction() handlers -- failing to do so causes the transaction connection to hang indefinitely)

(You MUST call knex.destroy() on graceful shutdown -- orphaned pools prevent the Node.js process from exiting)

</critical_requirements>

---

Examples

• Core Patterns -- Initialization, query builder, insert/update/delete, raw queries, TypeScript integration
• Schema & Migrations -- Schema builder, createTable, alterTable, migrations, seeds
• Transactions & Advanced -- Transactions, batch insert, subqueries, connection pooling, multi-tenancy

Additional resources:

• reference.md -- Query method cheat sheet, column types, pool options, anti-patterns, production checklist

---

Auto-detection: Knex, knex, knexfile, knex.raw, knex.schema, knex.transaction, knex.migrate, knex.seed, batchInsert, query builder, schema builder, SQL query builder, knex.fn.now, knex.ref, knex.destroy, pg, mysql2, sqlite3, better-sqlite3

When to use:

• Building SQL queries programmatically with a fluent API
• Database schema creation and modification (createTable, alterTable)
• Running and managing database migrations (up/down)
• Seeding development/test databases
• Wrapping multi-step database operations in transactions
• Writing raw SQL with safe parameter binding
• Batch inserting large datasets with chunking

Key patterns covered:

• Knex initialization with connection pool configuration
• Fluent query builder (select, where, join, orderBy, groupBy, having)
• Insert, update, delete with .returning() for PostgreSQL/MSSQL
• Schema builder (createTable, alterTable, column types, indexes, foreign keys)
• Migrations (knex migrate:make, up/down, transaction control)
• Seeds (knex seed:make, seed:run)
• Transactions with async/await and isolation levels
• Raw queries with ? value bindings and ?? identifier bindings
• Subqueries as callbacks or builder instances
• Batch insert with batchInsert() and chunking
• TypeScript table type augmentation
• Connection pool tuning (min, max, acquireTimeout, lifetime)

When NOT to use:

• You need a full ORM with model relationships, lifecycle hooks, and identity maps -- use your ORM solution instead
• You need database-specific features Knex doesn't abstract (e.g., PostgreSQL LISTEN/NOTIFY, MySQL fulltext indexes) -- use knex.raw() for those
• Your project already uses a different query layer or ORM and doesn't need a second one

---

Philosophy

Knex is a SQL query builder, not an ORM. The core principle: you write SQL, Knex just makes it safer and more portable.

Core principles:

1. One instance, one pool -- Initialize knex once. The instance manages a connection pool (tarn.js). Never create multiple knex instances pointing at the same database.
2. Parameterize everything -- Use ? bindings for values and ?? for identifiers. Never interpolate strings into queries.
3. Migrations are the source of truth -- Schema changes happen through migrations, not ad-hoc knex.schema calls in application code.
4. Transactions for consistency -- Any operation touching multiple tables or needing atomicity must be wrapped in knex.transaction().
5. Knex is dialect-aware, not dialect-hiding -- Knex normalizes common SQL, but database-specific features (e.g., .returning() on PostgreSQL, ON DUPLICATE KEY on MySQL) must be handled per-dialect.

---

Core Patterns

Pattern 1: Knex Initialization

Initialize once per application. The knex instance manages a connection pool internally. See examples/core.md for full examples.

// Good Example -- Proper initialization with pool tuning
import knex from "knex";

const POOL_MIN = 0;
const POOL_MAX = 10;
const ACQUIRE_TIMEOUT_MS = 30_000;

function createDatabase() {
  const connectionString = process.env.DATABASE_URL;
  if (!connectionString) {
    throw new Error("DATABASE_URL environment variable is required");
  }

  return knex({
    client: "pg",
    connection: connectionString,
    pool: { min: POOL_MIN, max: POOL_MAX },
    acquireConnectionTimeout: ACQUIRE_TIMEOUT_MS,
  });
}

export { createDatabase };

Why good: Single instance, environment variable for connection string, pool min: 0 releases idle connections, named constants

// Bad Example -- Multiple instances, hardcoded config
import knex from "knex";

function getUsers() {
  const db = knex({ client: "pg", connection: "postgres://localhost/mydb" });
  return db("users").select("*");
  // Connection pool leaked -- db.destroy() never called
}

Why bad: Creates a new pool per call (leaks connections), hardcoded connection string, select("*") fetches unnecessary columns

---

Pattern 2: Query Builder Basics

Fluent API for building SELECT queries. See examples/core.md for joins, groupBy, having.

// Good Example -- Typed query with explicit columns
const ACTIVE_STATUS = "active";
const PAGE_SIZE = 25;

const users = await db<User>("users")
  .select("id", "name", "email")
  .where("status", ACTIVE_STATUS)
  .orderBy("created_at", "desc")
  .limit(PAGE_SIZE);

Why good: Explicit column selection, typed result, named constants for status and page size

// Bad Example -- select(*) with string interpolation
const users = await db("users").select("*").whereRaw(`status = '${status}'`); // SQL INJECTION

Why bad: select("*") fetches unnecessary data, string interpolation in whereRaw creates SQL injection vulnerability

---

Pattern 3: Insert / Update / Delete with Returning

.returning() works on PostgreSQL, MSSQL, CockroachDB, and SQLite 3.35+. MySQL ignores it silently. See examples/core.md.

// Good Example -- Insert with returning (PostgreSQL)
const [inserted] = await db("users")
  .insert({ name: "Alice", email: "[email protected]" })
  .returning(["id", "created_at"]);

// Good Example -- Update with returning
const [updated] = await db("users")
  .where("id", userId)
  .update({ name: newName, updated_at: db.fn.now() })
  .returning(["id", "name", "updated_at"]);

Why good: .returning() avoids a separate SELECT, db.fn.now() uses database-native timestamp

// Bad Example -- Forgetting returning() on PostgreSQL
await db("users").insert({ name: "Alice" });
// Returns [] (empty array) on PostgreSQL, not the inserted data
// Developer expects the inserted row but gets a useless number

Why bad: Without .returning(), PostgreSQL insert returns row count (not data), forcing an extra SELECT query

---

Pattern 4: Raw Queries with Safe Bindings

Use ? for value bindings and ?? for identifier bindings. See examples/core.md.

// Good Example -- Parameterized raw query
const MIN_ORDER_COUNT = 5;

const results = await db.raw(
  `SELECT ??, COUNT(*) as order_count
   FROM ??
   WHERE ?? > ?
   GROUP BY ??
   HAVING COUNT(*) >= ?`,
  [
    "users.id",
    "orders",
    "orders.created_at",
    cutoffDate,
    "users.id",
    MIN_ORDER_COUNT,
  ],
);

Why good: ?? for identifiers, ? for values, all user input parameterized

// Bad Example -- String concatenation in raw query
const results = await db.raw(`SELECT * FROM users WHERE name = '${name}'`);
// SQL INJECTION: name = "'; DROP TABLE users; --"

Why bad: String interpolation allows SQL injection, attacker can execute arbitrary SQL

---

Pattern 5: Transactions

Wrap multi-step operations in transactions. Return or await the promise -- otherwise the connection hangs. See examples/transactions-advanced.md.

// Good Example -- Async/await transaction
const result = await db.transaction(async (trx) => {
  const [order] = await trx("orders")
    .insert({ user_id: userId, total: amount })
    .returning("id");

  await trx("order_items").insert(
    items.map((item) => ({ order_id: order.id, ...item })),
  );

  await trx("inventory")
    .whereIn(
      "product_id",
      items.map((i) => i.product_id),
    )
    .decrement("quantity", 1);

  return order;
});
// Transaction auto-commits on success, auto-rolls-back on thrown error

Why good: All operations atomic, auto-commit on success, auto-rollback on error, returns value from transaction

// Bad Example -- Forgetting to return/await inside transaction
await db.transaction((trx) => {
  trx("orders").insert({ user_id: userId }); // NOT returned/awaited
  trx("items").insert({ order_id: 1 }); // NOT returned/awaited
  // Transaction handler returns undefined -- trx NEVER commits or rolls back
  // Connection hangs until acquireConnectionTimeout fires
});

Why bad: Without returning a promise, Knex cannot detect completion, transaction hangs indefinitely consuming a pool connection

---

Pattern 6: Schema Builder

Create and modify tables. Use in migrations, not application code. See examples/schema-migrations.md.

// Good Example -- Migration creating a table
export async function up(knex: Knex): Promise<void> {
  await knex.schema.createTable("orders", (table) => {
    table.increments("id").primary();
    table
      .integer("user_id")
      .unsigned()
      .notNullable()
      .references("id")
      .inTable("users")
      .onDelete("CASCADE");
    table.decimal("total", 10, 2).notNullable();
    table
      .enum("status", ["pending", "paid", "shipped", "cancelled"])
      .notNullable()
      .defaultTo("pending");
    table.timestamps(true, true); // created_at, updated_at with defaults
    table.index(["user_id", "status"]);
  });
}

export async function down(knex: Knex): Promise<void> {
  await knex.schema.dropTable("orders");
}

Why good: Foreign key with cascade, composite index, enum constraint, timestamps with defaults, reversible down migration

---

<decision_framework>

Decision Framework

Knex Method Selection

What kind of database operation?
-- SELECT query -> db("table").select().where()
-- INSERT -> db("table").insert(data).returning()
-- UPDATE -> db("table").where().update(data).returning()
-- DELETE -> db("table").where().del()
-- Schema change -> db.schema.createTable() / .alterTable() (in migrations only)
-- Complex SQL -> db.raw("SQL", bindings)
-- Batch insert -> db.batchInsert("table", rows, chunkSize)
-- Multi-table atomic write -> db.transaction(async (trx) => { ... })

When to Use Raw Queries

Can the query builder express this?
-- YES -> Use the query builder (portable, type-safe)
-- NO -> Does it use database-specific syntax?
    -- YES -> Use db.raw() with parameterized bindings
    -- NO -> Is it a performance-critical query needing exact SQL?
        -- YES -> Use db.raw() with parameterized bindings
        -- NO -> File an issue or use a subquery callback

Transaction vs No Transaction

Does this operation modify multiple tables?
-- YES -> Use db.transaction()
Does this read need snapshot isolation?
-- YES -> Use db.transaction({ isolationLevel: "repeatable read" })
Is this a single INSERT/UPDATE/DELETE?
-- YES -> No transaction needed (single statement is atomic)

.returning() Behavior by Database

Which database are you targeting?
-- PostgreSQL -> .returning() works, returns array of objects
-- MSSQL -> .returning() works, returns array of objects
-- SQLite 3.35+ -> .returning() works
-- MySQL -> .returning() is silently ignored, insert returns [insertId]
-- Oracle -> .returning() works

</decision_framework>

---

<red_flags>

RED FLAGS

High Priority Issues:

• String interpolation in knex.raw() or .whereRaw() -- SQL injection vulnerability; always use ? / ?? bindings
• Creating multiple knex instances pointing at the same database -- leaks connection pools, exhausts database connections
• Not returning/awaiting the promise inside knex.transaction() handler -- transaction connection hangs indefinitely
• Missing knex.destroy() on shutdown -- orphaned pool prevents process exit, connections leak
• Running knex.schema calls in application code instead of migrations -- schema state becomes unpredictable across environments

Medium Priority Issues:

• Using select("*") in production queries -- fetches unnecessary data, increases memory usage, breaks when columns are added
• Forgetting .returning() on PostgreSQL inserts -- returns empty array [] instead of inserted data
• Not setting pool min: 0 -- default min: 2 keeps stale connections alive during low-traffic periods
• Missing WHERE clause on .update() or .del() -- updates/deletes ALL rows in the table
• Using KEYS-style patterns without pagination -- db("table").select() with no limit loads entire table into memory

Common Mistakes:

• Expecting .returning() to work on MySQL -- it is silently ignored; use insertId from the result instead
• Using .timeout() on the query without { cancel: true } -- times out the Node.js side but the query keeps running on the database server
• Running migrations with disableTransactions: true and assuming rollback works -- without a transaction, a failed migration leaves the database in a partial state
• Assuming knex.schema.hasTable() and knex.schema.createTable() are atomic -- another process can create the table between the check and the create
• Calling trx.commit() or trx.rollback() AND returning a promise -- double-completion causes unpredictable behavior

Gotchas & Edge Cases:

• knex.raw() returns a { rows, fields } object on PostgreSQL but a flat array on MySQL -- access .rows for PostgreSQL or destructure accordingly
• .timestamps(true, true) creates created_at and updated_at with defaultTo(knex.fn.now()) -- but updated_at is NOT automatically updated on row changes; you must set it yourself in UPDATE queries or use a database trigger
• .first() returns undefined (not null) when no row matches -- check with if (!result) not if (result === null)
• knex.batchInsert() wraps all chunks in a single transaction by default -- if one chunk fails, all previous chunks are rolled back
• Column names in .returning() must match the database column names exactly (case-sensitive on PostgreSQL)
• .whereIn("id", []) with an empty array generates WHERE 1 = 0 (always false) -- Knex handles it but it can be surprising in logs
• Migrations run in filename-sorted order -- ensure timestamps are consistent (don't mix manual names with generated timestamps)
• knex.fn.now() is evaluated by the database server, not Node.js -- useful for consistency but means you can't mock it in tests without stubbing the query

</red_flags>

---

<critical_reminders>

CRITICAL REMINDERS

All code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering, import type, named constants)

(You MUST initialize the knex instance ONCE per application and reuse it -- creating multiple instances leaks connection pools)

(You MUST use parameterized bindings (? for values, ?? for identifiers) in ALL knex.raw() calls -- string interpolation causes SQL injection)

(You MUST return or await the promise inside knex.transaction() handlers -- failing to do so causes the transaction connection to hang indefinitely)

(You MUST call knex.destroy() on graceful shutdown -- orphaned pools prevent the Node.js process from exiting)

Failure to follow these rules will cause SQL injection vulnerabilities, connection pool exhaustion, hanging transactions, and zombie processes.

</critical_reminders>