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Kysely’s plugin system allows you to intercept and modify queries before execution and transform results after execution. Plugins enable features like automatic column name transformation, query logging, and custom data parsing.

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The KyselyPlugin Interface

All plugins implement the KyselyPlugin interface: 
import type {
  KyselyPlugin,
  PluginTransformQueryArgs,
  PluginTransformResultArgs,
  QueryResult,
  RootOperationNode,
  UnknownRow
} from 'kysely'

interface KyselyPlugin {
  /**
   * This is called for each query before it is executed.
   * You can modify the query by transforming its OperationNode tree.
   */
  transformQuery(args: PluginTransformQueryArgs): RootOperationNode

  /**
   * This method is called for each query after it has been executed.
   * You can modify the result and return the modified result.
   */
  transformResult(
    args: PluginTransformResultArgs
  ): Promise<QueryResult<UnknownRow>>
}

interface PluginTransformQueryArgs {
  readonly queryId: QueryId
  readonly node: RootOperationNode
}

interface PluginTransformResultArgs {
  readonly queryId: QueryId
  readonly result: QueryResult<UnknownRow>
}

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Installing Plugins

Plugins are registered when creating a Kysely instance: 
import { Kysely, CamelCasePlugin } from 'kysely'

const db = new Kysely<Database>({
  dialect: new PostgresDialect({ /* ... */ }),
  plugins: [new CamelCasePlugin()]
})
Or add them to an existing instance: 
const dbWithPlugin = db.withPlugin(new CamelCasePlugin())

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Built-in Plugins

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CamelCasePlugin

Converts snake_case database identifiers to camelCase in JavaScript: 
import { CamelCasePlugin, Kysely, SqliteDialect } from 'kysely'
import Database from 'better-sqlite3'

// Define your database in camelCase
interface CamelCasedDatabase {
  userMetadata: {
    firstName: string
    lastName: string
    createdAt: Date
  }
}

const db = new Kysely<CamelCasedDatabase>({
  dialect: new SqliteDialect({
    database: new Database(':memory:'),
  }),
  plugins: [new CamelCasePlugin()],
})

// Use camelCase in your code
const person = await db.selectFrom('userMetadata')
  .where('firstName', '=', 'Jennifer')
  .select(['firstName', 'lastName'])
  .executeTakeFirst()

if (person) {
  console.log(person.firstName) // Typed as string
}
Generated SQL: 
select "first_name", "last_name" 
from "user_metadata" 
where "first_name" = ?
Options: 
new CamelCasePlugin({
  // Transform to UPPER_CASE instead of snake_case
  upperCase: true,
  
  // Add underscore before digits: foo12Bar => foo_12_bar
  underscoreBeforeDigits: true,
  
  // Add underscore between uppercase letters: fooBAR => foo_b_a_r
  underscoreBetweenUppercaseLetters: true,
  
  // Don't transform nested object keys
  maintainNestedObjectKeys: false,
})

​
DeduplicateJoinsPlugin

Automatically deduplicates identical JOIN clauses: 
import { DeduplicateJoinsPlugin } from 'kysely'

const db = new Kysely<Database>({
  dialect: /* ... */,
  plugins: [new DeduplicateJoinsPlugin()]
})

// These joins are identical and will be deduplicated
await db
  .selectFrom('person')
  .innerJoin('pet', 'pet.owner_id', 'person.id')
  .innerJoin('pet', 'pet.owner_id', 'person.id') // Deduplicated
  .selectAll()
  .execute()

​
ParseJSONResultsPlugin

Automatically parses JSON columns: 
import { ParseJSONResultsPlugin } from 'kysely'

const db = new Kysely<Database>({
  dialect: /* ... */,
  plugins: [new ParseJSONResultsPlugin()]
})

interface Database {
  person: {
    id: number
    metadata: { tags: string[]; score: number } // JSON column
  }
}

const person = await db
  .selectFrom('person')
  .selectAll()
  .executeTakeFirst()

// metadata is automatically parsed
person?.metadata.tags // string[]

​
Creating Custom Plugins

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Basic Example: Query Logger

import type {
  KyselyPlugin,
  PluginTransformQueryArgs,
  PluginTransformResultArgs,
  QueryResult,
  RootOperationNode,
  UnknownRow
} from 'kysely'

class QueryLoggerPlugin implements KyselyPlugin {
  transformQuery(args: PluginTransformQueryArgs): RootOperationNode {
    console.log('Executing query:', args.node)
    return args.node
  }

  async transformResult(
    args: PluginTransformResultArgs
  ): Promise<QueryResult<UnknownRow>> {
    console.log('Query returned', args.result.rows.length, 'rows')
    return args.result
  }
}

// Usage
const db = new Kysely<Database>({
  dialect: /* ... */,
  plugins: [new QueryLoggerPlugin()]
})

​
Advanced Example: Timing Plugin

class QueryTimingPlugin implements KyselyPlugin {
  private timings = new WeakMap<any, number>()

  transformQuery(args: PluginTransformQueryArgs): RootOperationNode {
    this.timings.set(args.queryId, Date.now())
    return args.node
  }

  async transformResult(
    args: PluginTransformResultArgs
  ): Promise<QueryResult<UnknownRow>> {
    const startTime = this.timings.get(args.queryId)
    
    if (startTime) {
      const duration = Date.now() - startTime
      console.log(`Query executed in ${duration}ms`)
    }
    
    return args.result
  }
}

​
Sharing Data Between transformQuery and transformResult

Use a WeakMap with queryId as the key: 
import type {
  KyselyPlugin,
  QueryResult,
  RootOperationNode,
  UnknownRow
} from 'kysely'

interface MyData {
  startTime: number
  queryType: string
}

class MyPlugin implements KyselyPlugin {
  private data = new WeakMap<any, MyData>()

  transformQuery(args: PluginTransformQueryArgs): RootOperationNode {
    const myData: MyData = {
      startTime: Date.now(),
      queryType: args.node.kind
    }

    // Store data for this query
    this.data.set(args.queryId, myData)

    return args.node
  }

  async transformResult(
    args: PluginTransformResultArgs
  ): Promise<QueryResult<UnknownRow>> {
    // Retrieve data for this query
    const myData = this.data.get(args.queryId)

    if (myData) {
      console.log(`${myData.queryType} took ${Date.now() - myData.startTime}ms`)
    }

    return args.result
  }
}
Always use WeakMap instead of Map for storing query data. transformQuery is not always matched by transformResult, which could cause memory leaks with strong references.

​
Transforming the Query AST

Use an OperationNodeTransformer to modify query structure: 
import {
  OperationNodeTransformer,
  SelectQueryNode,
  LimitNode
} from 'kysely'

class AutoLimitPlugin implements KyselyPlugin {
  transformQuery(args: PluginTransformQueryArgs): RootOperationNode {
    const transformer = new AutoLimitTransformer()
    return transformer.transformNode(args.node)
  }

  async transformResult(
    args: PluginTransformResultArgs
  ): Promise<QueryResult<UnknownRow>> {
    return args.result
  }
}

class AutoLimitTransformer extends OperationNodeTransformer {
  protected override transformSelectQuery(
    node: SelectQueryNode
  ): SelectQueryNode {
    // Add LIMIT 1000 if no limit exists
    if (!node.limit) {
      return {
        ...super.transformSelectQuery(node),
        limit: LimitNode.create(1000)
      }
    }
    
    return super.transformSelectQuery(node)
  }
}

​
Plugin Execution Order

Plugins execute in the order they’re registered: 
const db = new Kysely<Database>({
  dialect: /* ... */,
  plugins: [
    new PluginA(), // Runs first
    new PluginB(), // Runs second
    new PluginC()  // Runs last
  ]
})
For transformQuery, plugins run in order: 
Query → PluginA → PluginB → PluginC → Database
For transformResult, plugins run in reverse: 
Database → PluginC → PluginB → PluginA → Result

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Dynamic Plugin Management

// Add a plugin
const dbWithPlugin = db.withPlugin(new MyPlugin())

// Add at front (executes first)
const dbWithPluginFirst = db.withPluginAtFront(new MyPlugin())

// Remove all plugins
const dbWithoutPlugins = db.withoutPlugins()

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Real-World Plugin Examples

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Soft Delete Plugin

class SoftDeletePlugin implements KyselyPlugin {
  transformQuery(args: PluginTransformQueryArgs): RootOperationNode {
    const transformer = new SoftDeleteTransformer()
    return transformer.transformNode(args.node)
  }

  async transformResult(
    args: PluginTransformResultArgs
  ): Promise<QueryResult<UnknownRow>> {
    return args.result
  }
}

class SoftDeleteTransformer extends OperationNodeTransformer {
  protected override transformSelectQuery(
    node: SelectQueryNode
  ): SelectQueryNode {
    // Automatically add WHERE deleted_at IS NULL
    // Implementation details omitted for brevity
    return super.transformSelectQuery(node)
  }
}

​
Row-Level Security Plugin

class TenantPlugin implements KyselyPlugin {
  constructor(private tenantId: string) {}

  transformQuery(args: PluginTransformQueryArgs): RootOperationNode {
    // Add WHERE tenant_id = this.tenantId to all queries
    const transformer = new TenantTransformer(this.tenantId)
    return transformer.transformNode(args.node)
  }

  async transformResult(
    args: PluginTransformResultArgs
  ): Promise<QueryResult<UnknownRow>> {
    return args.result
  }
}

​
Best Practices

Keep plugins focused on a single responsibility. Create multiple small plugins rather than one large plugin.
Always return a value from transformQuery and transformResult. Returning undefined will cause errors.
Plugins that modify the query structure can break type safety. Test thoroughly when creating AST-transforming plugins.
Use WeakMap with queryId to share data between transformQuery and transformResult to prevent memory leaks.

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