sequelize/sequelize · error · Error

Model ${model.name} has already been initialized. Models can

Error message

Model ${model.name} has already been initialized. Models can only belong to one Sequelize instance. Registering the same model with multiple Sequelize instances is not yet supported. Please see https://github.com/sequelize/sequelize/issues/15389

What it means

Thrown by registerModelDefinition when a Model class already has an associated ModelDefinition (model-definition.ts:867). Sequelize ties a Model class to a single Sequelize instance via a WeakMap; re-initializing the same class on another instance (or re-calling init on the same class) is rejected because the prior binding still exists.

Source

Thrown at packages/core/src/model-definition.ts:868

  isParanoid(): boolean {
    return Boolean(this.timestampAttributeNames.deletedAt);
  }
}

const modelDefinitionListeners = new Set<(model: ModelStatic) => void>();
export function listenForModelDefinition(callback: (model: ModelStatic) => void): void {
  modelDefinitionListeners.add(callback);
}

const modelDefinitions = new WeakMap</* model class */ Function, ModelDefinition<any>>();

export function registerModelDefinition<M extends Model>(
  model: ModelStatic<M>,
  modelDefinition: ModelDefinition<M>,
): void {
  if (modelDefinitions.has(model)) {
    throw new Error(
      `Model ${model.name} has already been initialized. Models can only belong to one Sequelize instance. Registering the same model with multiple Sequelize instances is not yet supported. Please see https://github.com/sequelize/sequelize/issues/15389`,
    );
  }

  modelDefinitions.set(model, modelDefinition);

  for (const listener of modelDefinitionListeners) {
    listener(model);
  }
}

export function removeModelDefinition(model: ModelStatic): void {
  modelDefinitions.delete(model);
}

export function hasModelDefinition(model: ModelStatic): boolean {
  return modelDefinitions.has(model);
}

View on GitHub (pinned to 7e1deec499)

Solutions

Define the Model on only one Sequelize instance; for multi-tenant, create separate Model subclasses per instance.
Call removeModelDefinition(model) (or model.sequelize = null pattern) before re-initializing when intentionally rebinding.
In HMR/dev setups, isolate model definitions per reload or reset module registry between reloads.

Example fix

// before
User.init(attrs, { sequelize: instanceA });
User.init(attrs, { sequelize: instanceB }); // throws
// after
const UserA = class extends Model {};
const UserB = class extends Model {};
UserA.init(attrs, { sequelize: instanceA });
UserB.init(attrs, { sequelize: instanceB });

Defensive patterns

Strategy: validation

Validate before calling

import { hasModelDefinition, removeModelDefinition } from '@sequelize/core';
function safeInit(Model, attrs, opts) {
  if (hasModelDefinition(Model)) {
    throw new Error(`${Model.name} already bound; create a subclass for a new instance`);
  }
  Model.init(attrs, opts);
}

Type guard

import { hasModelDefinition } from '@sequelize/core';
function isModelBound(Model): boolean { return hasModelDefinition(Model); }

Prevention

Create distinct Model subclasses per Sequelize instance in multi-tenant apps.
In HMR/dev, clear the module cache or call removeModelDefinition before re-init.
Avoid calling Model.init twice on the same class.

When it happens

Trigger: Calling User.init(...) twice; sharing a Model class file across two Sequelize instances in the same process; hot-reloading (HMR) in dev servers that re-evaluates the model module while the old class object is retained.

Common situations: Multi-tenant setups that create multiple Sequelize instances; test suites that reuse imported models across connections; Jest/Vite HMR re-running module initialization.

Related errors

AI-assisted analysis of sequelize/sequelize@7e1deec499 (2026-08-03). Data as JSON: /data/errors/6314ce04c96fde9f.json. Report an issue: GitHub.