Postgres: Creating Relationships

Introduction

A relationship from one table/view to another can be created by defining a link between a column of the table/view to a column of the other table/view.

Typically, relationships are defined using foreign-key constraints. But in some cases, it might not be possible to use foreign-key constraints to create the relation. For example, while trying to create a relationship involving a view as foreign-keys can't be created on views.

Using foreign keys

Say we created two tables, authors(id, name) and articles(id, title, content, rating, author_id).

Let us now connect these tables to enable nested queries using a foreign-key:

Step 1: Add foreign-key constraint

Let's add a foreign-key constraint to the author_id column in the articles table.

Console

In the console, navigate to the Modify tab of the articles table. Click the Add button in the Foreign Keys section and configure the author_id column as a foreign-key for the id column in the authors table:

CLI

Create a migration manually and add the following SQL statement to the up.sql file:

ALTER TABLE articles
ADD FOREIGN KEY (author_id) REFERENCES authors(id);

Add the following statement to the down.sql file in case you need to roll back the above statement:

ALTER TABLE articles
DROP CONSTRAINT articles_author_id_fkey;

Apply the migration by running:

hasura migrate apply

API

You can add a foreign-key constraint using the run_sql schema API:

POST /v2/query HTTP/1.1
Content-Type: application/json
X-Hasura-Role: admin

{
  "type": "run_sql",
  "args": {
    "source": "<db_name>",
    "sql": "ALTER TABLE articles ADD FOREIGN KEY (author_id) REFERENCES authors(id);"
  }
}

Step 2: Create an object relationship

Each article has one author. This is an object relationship.

Console

The console infers potential relationships using the foreign-key created above and recommends these in the Relationships tab of the articles table.

Add an object relationship named author for the articles table as shown here:

CLI

You can add an object relationship in the tables.yaml file inside the metadata directory:

- table:
    schema: public
    name: articles
  object_relationships:
    - name: author
      using:
        foreign_key_constraint_on: author_id
- table:
    schema: public
    name: authors

Apply the metadata by running:

hasura metadata apply

API

You can create an object relationship by using the pg_create_object_relationship metadata API:

POST /v1/metadata HTTP/1.1
Content-Type: application/json
X-Hasura-Role: admin

{
  "type": "pg_create_object_relationship",
  "args": {
    "source": "<db_name>",
    "table": "articles",
    "name": "author",
    "using": {
      "foreign_key_constraint_on": "author_id"
    }
  }
}

We can now run a nested object query that is based on this object relationship.

Fetch a list of articles and each article's author:

GraphiQL

Query Variables

Request Headers

Step 3: Create an array relationship

An author can write multiple articles. This is an array relationship.

You can add an array relationship in the same fashion as an object relationship as shown above.

Console

On the console, add an array relationship named articles for the authors table as shown here:

We can now run a nested object query that is based on this array relationship.

CLI

You can add an array relationship in the tables.yaml file inside the metadata directory:

- table:
    schema: public
    name: articles
  object_relationships:
    - name: author
      using:
        foreign_key_constraint_on: author_id
- table:
    schema: public
    name: authors
  array_relationships:
    - name: articles
      using:
        foreign_key_constraint_on:
          column: author_id
          table:
            schema: public
            name: articles

Apply the metadata by running:

hasura metadata apply

API

You can create an array relationship by using the pg_create_array_relationship metadata API:

POST /v1/metadata HTTP/1.1
Content-Type: application/json
X-Hasura-Role: admin

{
  "type": "pg_create_array_relationship",
  "args": {
    "source": "<db_name>",
    "table": "authors",
    "name": "articles",
    "using": {
      "foreign_key_constraint_on" : {
        "table" : "articles",
        "column" : "author_id"
      }
    }
  }
}

Fetch a list of authors and a nested list of each author's articles:

GraphiQL

Query Variables

Request Headers

Using manual relationships

Let's say you have a table authors (id, name) and a view author_avg_rating (id, avg) which has the average rating of articles for each author.

Let us now create an object relationship called avg_rating from the authors table to the author_avg_rating view using a manual relationship:

Console

Step 1: Open the manual relationship section

• Open the console and navigate to the Data -> authors -> Relationships tab.
• Click on the Configure button:

Step 2: Define the relationship

The above step will open up a section as shown below:

In this case:

• Relationship Type will be: Object Relationship
• Relationship Name can be: avg_rating
• Reference will be: id -> author_avg_rating . id (similar to defining a foreign-key)

Step 3: Create the relationship

Now click on the Save button to create the relationship.

CLI

You can add a manual relationship in the tables.yaml file inside the metadata directory:

- table:
    schema: public
    name: articles
- table:
    schema: public
    name: authors
  object_relationships:
    - name: avg_rating
      using:
        manual_configuration:
          remote_table:
            schema: public
            name: author_average_rating
          column_mapping:
            id: author_id
- table:
    schema: public
    name: author_average_rating

Apply the metadata by running:

hasura metadata apply

API

You can add a manual relationship by using the pg_create_object_relationship metadata API:

POST /v1/metadata HTTP/1.1
Content-Type: application/json
X-Hasura-Role: admin

{
  "type": "pg_create_object_relationship",
  "args": {
    "source": "<db_name>",
    "table": "authors",
    "name": "avg_rating",
    "using": {
      "manual_configuration": {
        "remote_table": "author_average_rating",
        "column_mapping": {
          "id": "author_id"
        }
      }
    }
  }
}

We can now run a nested object query that is based on this object relationship.

Fetch a list of authors with the average rating of their articles:

GraphiQL

Query Variables

Request Headers

Tracking existing relationships inferred via foreign-keys

As mentioned in the Introduction section above, relationships can be inferred via foreign-keys that exist in your database:

Console

The console infers potential relationships using existing foreign-keys and recommends these on the Data -> Schema page

You can choose to track the relationships individually using the Track buttons or hit the Track all button to track all the inferred relationships in one go.

CLI

You can add relationships in the tables.yaml file inside the metadata directory:

- table:
    schema: public
    name: articles
  object_relationships:
    - name: author
      using:
        foreign_key_constraint_on: author_id
- table:
    schema: public
    name: authors
  array_relationships:
    - name: articles
      using:
        foreign_key_constraint_on:
          column: author_id
          table:
            schema: public
            name: articles

Apply the metadata by running:

hasura metadata apply

API

You can create multiple relationships by using the pg_create_object_relationship and the pg_create_array_relationship metadata APIs:

POST /v1/metadata HTTP/1.1
Content-Type: application/json
X-Hasura-Role: admin

{
  "type": "bulk",
  "args": [
    {
      "type": "pg_create_object_relationship",
      "args": {
        "source": "<db_name>",
        "table": "articles",
        "name": "author",
        "using": {
          "foreign_key_constraint_on": "author_id"
        }
      }
    },
    {
      "type": "pg_create_array_relationship",
      "args": {
        "source": "<db_name>",
        "table": "authors",
        "name": "articles",
        "using": {
          "foreign_key_constraint_on" : {
            "table" : "articles",
            "column" : "author_id"
          }
        }
      }
    }
  ]
}