Sequelize Associations: How a Misplaced belongsTo Broke Our Query (and How to Fix It)

Originally published on shorterloop.com.

As developers, we’ve all faced that moment of dread: your code should work, but instead, your logs fill with cryptic SQL errors. Recently, a team ran into a baffling Sequelize issue — a simple update query for a feedback system suddenly demanded a non-existent feedback_item_id field. Let’s dissect what went wrong, why, and how to avoid this trap.

module.exports = function (sequelize, DataTypes) {
  const feedbackItem = sequelize.define("idm_feedback_items", {
    // ... title, status, etc. ...
    status: {
      type: DataTypes.STRING,
      defaultValue: STATUS.active
    },
    user_id: DataTypes.INTEGER,  // Foreign key for user
    category_id: DataTypes.INTEGER, // Foreign key for category
  });

  feedbackItem.associate = (models) => {
    // 🚩 Problematic association!
    feedbackItem.belongsTo(models.idm_ideas_tags, {
      foreignKey: "feedback_item_id",
      as: "taggings"
    });

    // Correct many-to-many setup with tags
    feedbackItem.belongsToMany(models.tags, {
      through: models.idm_ideas_tags,
      foreignKey: "feedback_item_id",
      as: "tags"
    });
  };

  return feedbackItem;
};

Everything seemed fine — until they ran an update:

await feedback_items.update(
  { status: "deleted" },
  { where: { external_key: "IDEA-10", product_id: 1131 } }
);

Suddenly, their logs showed:

SELECT ..., `feedback_item_id` FROM `idm_feedback_items` WHERE ...;

The problem? The feedback_item_id column didn’t exist in their table. Why did Sequelize include it?

The Culprit: Misusing belongsTo

The root cause was an incorrect association. Let’s break it down:

1. The Offending Code

In the associate function:

feedbackItem.belongsTo(models.idm_ideas_tags, {
  foreignKey: "feedback_item_id", // 🚩 Red flag!
  as: "taggings"
});

2. Why This Broke

• idm_ideas_tags was a junction table for a many-to-many relationship between feedback items and tags.
• Using belongsTo incorrectly implied that idm_feedback_items had a direct foreign key feedback_item_id pointing to the junction table.
• Sequelize assumed feedback_item_id was a column in idm_feedback_items and included it in queries.

The Fix: Use belongsToMany for Junction Tables

Step 1: Remove the Incorrect Association

Delete the problematic belongsTo block:

// Remove this entirely!
// feedbackItem.belongsTo(models.idm_ideas_tags, { ... });

Step 2: Strengthen the Many-to-Many Setup

Ensure the relationship uses belongsToMany:

feedbackItem.belongsToMany(models.tags, {
  through: models.idm_ideas_tags, // Junction table
  foreignKey: "feedback_item_id", // Field in JUNCTION table
  otherKey: "tag_id",             // Field in JUNCTION table
  as: "tags"
});

How This Works:

• belongsToMany tells Sequelize the relationship is managed via a junction table.
• foreignKey and otherKey refer to columns in the junction table, not the main model.

Lessons Learned

1. belongsTo vs. belongsToMany

AssociationUse CaseForeign Key LocationbelongsToMany-to-One or One-to-OneCurrent model’s tablebelongsToManyMany-to-Many (via junction)Junction table

2. Common Pitfalls

• Assuming junction tables need direct associations: They don’t — use belongsToMany.
• Naming conflicts: Ensure foreign keys in associations don’t clash with existing columns.

3. Debugging Tips

• Log Sequelize queries: Enable logging: console.log to spot unexpected fields.
• Validate associations: Double-check foreignKey and through settings.

Best Practices for Sequelize Associations

1. Map Relationships Early: Sketch your schema to identify junction tables and relationship types.
2. Use Consistent Naming: Stick to conventions (e.g., feedback_item_id, not feedbackItemId).
3. Test Associations:

it("should load tags via junction table", async () => {
  const feedback = await FeedbackItem.findByPk(1, { include: "tags" });
  expect(feedback.tags).to.be.an("array");
});

Final Thoughts

Sequelize associations are powerful but require precision. Misusing belongsTo instead of belongsToMany is a common pitfall with junction tables. By understanding how these methods map to your schema, you’ll avoid unexpected SQL chaos.

Next time your query includes a mysterious field, check your associations — it might save hours of debugging!

Gotchas to Watch For:

• Accidental belongsTo on junction tables.
• Typos in foreign key names.
• Forgetting to define both sides of an association.

Further Reading:

• Sequelize Many-to-Many Guide

Engage: Ever battled unexpected fields in Sequelize? Share your war stories below! 💬

This article was originally published on shorterloop.com.