## Query Across SQL and NoSQL — Without Writing Code

MongoDB and PostgreSQL are often used together in modern applications — each bringing unique strengths. While PostgreSQL offers structured, relational storage, MongoDB gives flexibility through document-based collections.

But combining data from both in real time has always been a challenge. Traditionally, you’d have to:
- Write custom backend logic to merge data
- Create multiple API calls or aggregation pipelines
- Maintain sync through fragile ETL workflows

To solve this challenge, API Maker introduces the **Find & Join** feature.

## Why Use Find & Join for MongoDB and PostgreSQL

Modern applications often use a **polyglot database architecture** — leveraging **MongoDB** for unstructured or flexible data and **PostgreSQL** for structured, relational data.

While this separation is practical from a design and performance standpoint, it introduces a major challenge:

 **How do you fetch connected data across MongoDB and PostgreSQL in a single query** — without building and maintaining complex backend logic?

This is exactly where **API Maker’s Find & Join feature** shines.

It allows you to **perform deep, cross-database joins and filters between MongoDB and PostgreSQL** using a single, declarative JSON query — all through REST APIs.

This page provides information on using the Find & Join feature specifically with MongoDB and PostgreSQL.  

If you're looking for a general overview of the Find & Join feature, please refer to the - [Find & Join Feature](https://apimaker.dev/find-and-join)

## Schema Setup in Api Maker

To enable Find & Join operations between MongoDB and PostgreSQL, you need to define relationships in your table/collection schema using **API Maker’s Table schema.**

For a Detailed Information about the Schema Setup, you can refer to this page - [Schema Setup](https://docs.apimaker.dev/v1/docs/schema/schema.html)

Once the schema is defined, API Maker can automatically resolve joins at runtime — even across different database types.
### Defining the Relationship in Schema

Let’s say:
- You have a **`users`** table in **PostgreSQL**
- You have a **`profiles`** collection in **MongoDB**

You want to use an API that supports the **Find & Join** feature on the `profiles` collection (MongoDB). In this case, you'd define the relationship like this:
```typescript
user_id: <ISchemaProperty>{
    __type: EType.number,
    instance: "Postgres",
    database: "accounts_db",
	table: "users",
	column: "id"
}
```

Here what each field means: 

- **`__type: EType.number :`** Indicates that the `user_id` field is a number (to match the PostgreSQL `id` column type).
- **`instance: "Postgres" :`** Specifies the target database engine — in this case, PostgreSQL.
- **`database: "accounts_db" :`** The name of the PostgreSQL database where the `users` table resides.
- **`table: "users" :`** The relational table in PostgreSQL to which the `user_id` field will be linked.
- **`column: "id" :`** The specific column in the `users` table that the `user_id` field refers to.

With this schema defined, API Maker can:
- Automatically recognize and resolve the relationship between MongoDB and PostgreSQL
- Enable join queries like `user_id.name` directly in your REST API calls
- Eliminate the need for custom backend join logic — it's all handled for you at runtime

## Sample Query Examples :

Once you've defined relationships between your MongoDB collections and PostgreSQL tables in the schema, you can use the **Find & Join** feature directly in your API calls — without writing any backend logic.

API Maker supports the Find & Join Feature in both the :
- **GET requests** (using URL query parameters)
- **POST requests** (using request body for advanced querying)
### **Example Scenario**

You have:
- A `users` table in **PostgreSQL**
- A `profiles` collection in **MongoDB** with a field `user_id` referencing `users.id`

**1. GET API Example (Using URL Query Parameter) :**
Use the `find` Query parameter in your auto-generated GET API to filter across relationships.

**Request:** 
```http
GET /api/profiles?find={ "user_id.name": "Alice" }
```

This query fetches all `profiles` (from MongoDB) where the related `user` (from PostgreSQL) has the name `"Alice"`.

**2. POST API Example :**
POST APIs give you more flexibility — allowing you to send a `find` object along with additional options like `sort`, `limit`, and `deep`.

**Request:**
```http
POST /api/query/profiles
Content-Type: application/json

{
  "find": {
    "user_id.signup_date": { "$gt": "2024-01-01" },
    "user_id.role": "admin"
  },
  "limit": 10,
  "sort": {
    "user_id.signup_date": -1
  }
}
```

This will return up to 10 profiles linked to PostgreSQL users who signed up after January 1st, 2024 and have the role `"admin"` — sorted by most recent signup.

To explore all REST APIs that support **Find & Join**, check out the [auto-generated REST APIs](https://docs.apimaker.dev/v1/docs/apis-all/generated-apis/auto-generated-get-all-api.html) and [schema-based REST APIs](https://docs.apimaker.dev/v1/docs/apis-all/schema-apis/auto-generated-schema-based-get-all-api.html) available on the [API Maker Docs Page](https://docs.apimaker.dev/).
## Comparison to Traditional Methods & Other Platforms

Joining data between **MongoDB** and **PostgreSQL** has traditionally been a complex, manual process for developers.

**Traditional Methods:**
Before tools like API Maker, developers had to:
- Write custom backend code to connect and query both databases
- Manually merge results at the application level
- Build and maintain fragile ETL pipelines
- Handle sync issues and performance bottlenecks

These methods are time-consuming and hard to scale — especially when relationships become more complex.

How API Maker Simplifies Cross Database Joins and Conditional Filtering :
- **No backend logic** required
- **Schema-based joins** resolved automatically at runtime
- **Real-time querying** across different databases
- Simple REST API interface — easy to use from frontend or backend

### Compared to Other Platforms 

| Feature                       | **API Maker** | **Appwrite** | **Supabase** | **Firebase**   |
| ----------------------------- | ------------- | ------------ | ------------ | -------------- |
| Cross-DB Joins (Mongo + SQL)  | Yes           | No           | No           | No             |
| N-Level Nested Joins          | Unlimited     | No           | Basic        | No             |
| REST Support for Join Queries | Full          | Partial      | Yes          | Firestore only |
| Schema-Based Join Logic       | Yes           | No           | Limited      | No             |
| Works with Mongo + PostgreSQL | Yes           | No           | No           | No             |

**API Maker** is the only platform among these that offers **real-time, schema-aware, cross database joins** between MongoDB and PostgreSQL — with **no backend logic required**.

## FAQ's 

1. **Can I join MongoDB collections with PostgreSQL tables in both directions?**
	Yes. As long as the relationship is defined in your schema, you can perform joins in either direction — **`MongoDB`** → **`PostgreSQL`** or **`PostgreSQL`** → **`MongoDB`** — using the same query structure.

2. **How do I filter on PostgreSQL fields from within a MongoDB collection?**
	You can use nested dot notation based on your schema definition. 
	For example: `{ "user_id.role": "admin" }`.
	In this case, `user_id` is a reference from the MongoDB `profiles` collection to the PostgreSQL `users` table.

3. **Is performance impacted by cross-database joins?**
	API Maker optimizes cross-database joins at runtime.
	For best performance, make sure to **index the join keys** in both MongoDB and PostgreSQL.