DOCS  vx.y

Support

Part IV: Add relationships

Exploiting relationships in your data

The data microservice allows you to set up different relationships between data, which enables you to make more complex and interesting queries to fetch data.

For example:

// Normal output of select query
[{
   "id": 1,
   "title": "My first article",
   "content": "Lots of content...",
   "author_id": 3
}]

// Output of select query using a relationship
[{
   "id": 1,
   "title": "My first article",
   "content": "Lots of content...",
   "author": {
      "name": "Ramu"
      "id": 3
   }
}]

Let’s look at the different relationships we have in our data models:

• author:
  • has multiple articles written by them
• article:
  • has one author
  • has multiple comments
  • has multiple likes
• comment:
  • has one article on which it was posted
• like:
  • has one article that has been liked

These relationships can be captured by foreign key constraints. If we were to represent rows of our table in JSON, as objects, then we can express these relationships as nested arrays or objects. E.g.: Every author object can have a key called articles which is an array of article objects. Similarly, every article object can have a key called author which is an author object.

Let’s see how these relationships are established:

Table	Relationship	Type	Established by
author	articles	array	article::author_id -> author::id
article	author	object	article::author_id -> author::id
article	comments	array	comment::article_id -> article::id
article	likes	array	like::article_id -> article::id
comment	article	object	comment::article_id -> article::id
like	article	object	like::article_id -> article::id

Creating relationships

You can create relationships for tables via the API console.

By default the object relationship author for the article table has already been created. Click on the article table and navigate to the Relationships tab.

You’ll see an entry in object relationships for author_id -> author::id:

Let’s say you wish to add the array relationship comments for the article table. You’ll see entries under suggested relationships, if you had added all the foreign key constraints that were mentioned in the previous part. Click on Add to add a new array relationship and name the relationship comments:

The relationship is created:

You can create the other relationships similarly.

Queries using relationships

To obtain the author’s name from the article table, we can write the following query:

query fetch_article {
   article {
     title
     author {
      name
     }
   }
}

The same syntax can be used to obtain the titles of all articles across all authors.

query fetch_author {
   author {
     name
     articles {
      title
     }
   }
}

You can use relationships inside where clauses. For example, if we wish to only fetch articles having a rating of 5 by the author with the name Warren, we could use:

query fetch_article {
   article (where: {rating: {_eq: 5} author: {name: {_eq: "Warren"}}} ) {
     id
     title
   }
}

Let’s fetch authors who have not written an article with a rating that is lower than 3:

query fetch_author {
   author (where: {_not: {articles: {rating: {_lte: 3}}}} ) {
     name
   }
}

As you probably guessed, relationships can be nested. Let’s get all authors, with their articles, with their comments.

query fetch_article {
   author {
     name
     articles {
       title
       comments {
         comment
       }
     }
   }
}

We can also use where, limit, offset inside array relationships. Let’s say we want to fetch all authors and only their articles having a rating that is higher than 2:

query fetch_author {
   author {
     name
     articles (where: {rating: {_gte: 2}}) {
      title
     }
   }
}

Next: Add permissions and access control

Next, head to Part V: Access control.

« previous | next »

Was this page helpful?

Submit

Thank you for your feedback!

Want to contribute or report missing content? Check out the github repo for docs.
Powered by Sphinx. Copyright © 2019 Hasura.