# Translation strategy

{% hint style="success" %}
This is the official documentation of the `@forestadmin/agent` Node.js agent.
{% endhint %}

### Steps

Creating a custom data source will require you to work on the 3 following steps:

1. Declare the structure of the data
2. Declare the API capabilities
3. Code a translation layer

### Minimal example

```javascript
const {
  BaseCollection,
  BaseDataSource,
} = require('@forestadmin/datasource-toolkit');
const axios = require('axios'); // client for the target API

// The real work is in writing this module
// Expect a full featured query translation module to be over 1000 LOCs
const QueryGenerator = require('./forest-query-translation');

/** Minimal implementation of a readonly data source */
class MyCollection extends BaseCollection {
  constructor(dataSource) {
    // Set name of the collection once imported
    super('myCollection', dataSource);

    this.addField('id', {
      // Structure
      type: 'Column',
      columnType: 'Number',
      isPrimaryKey: true,
      isReadOnly: true, // field is readonly

      // As we are using the query translation strategy => define capabilities
      filterOperators: new Set(), // field is not filterable
      isSortable: false, // field is not sortable
    });

    this.addField('title', {
      type: 'Column',
      columnType: 'String',
      isReadOnly: true,
      filterOperators: new Set(),
      isSortable: false,
    });
  }

  async list(caller, filter, projection) {
    const params = QueryGenerator.generateListQueryString(filter, projection);
    const response = axios.get('https://my-api/my-collection', { params });

    return response.body.items;
  }

  async aggregate(caller, filter, aggregation, limit) {
    const params = QueryGenerator.generateAggregateQueryString(
      filter,
      aggregation,
      limit,
    );
    const response = axios.get('https://my-api/my-collection', { params });

    return response.body.items;
  }
}

class MyDataSource extends BaseDataSource {
  constructor() {
    super();
    this.addCollection(new MyCollection(this)); // For all your collections
  }
}

module.exports = MyDataSource;
```

```javascript
const MyDataSource = require('./datasource');

const myDataSourceFactory = async () => new MyDataSource();

const agent = createAgent(options).addDataSource(myDataSourceFactory);
```

### Read more

Implementing a data source using the "query translation" strategy is an advanced concept: you will need to have a deep understanding of Forest Admin internals.

This strategy is a good match when writing data sources for full-featured databases.

Before starting, it is highly advised to read and understand the following sections:

* [Data Model Typing](/developer-guide-agents-nodejs/under-the-hood/data-model/typing.md)
* [Data Model Relationships](/developer-guide-agents-nodejs/under-the-hood/data-model/relationships.md)
* [Query interface](/developer-guide-agents-nodejs/data-sources/getting-started/queries.md)
* [Fields and projections](/developer-guide-agents-nodejs/data-sources/getting-started/queries/fields-projections.md)
* [Filters](/developer-guide-agents-nodejs/data-sources/getting-started/queries/filters.md)
* [Aggregations](/developer-guide-agents-nodejs/data-sources/getting-started/queries/aggregations.md)


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.forestadmin.com/developer-guide-agents-nodejs/data-sources/custom/translation.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.