Computed fields

What are computed fields?

Computed fields are virtual values or objects that are dynamically computed and can be queried along with a table’s columns. Computed fields are computed when requested for via custom SQL functions (a.k.a. stored procedures) using other columns of the table and other custom inputs if needed.

Note

Computed fields are only exposed over the GraphQL API and the database schema is not modified on addition of a computed field.

Supported SQL functions

Only functions which satisfy the following constraints can be added as a computed field to a table. (terminology from Postgres docs):

  • Function behaviour: ONLY STABLE or IMMUTABLE
  • Argument modes: ONLY IN
  • Table Argument: One input argument with a table row type
  • Return type: Either SETOF <table-name> or BASE type

Note

Functions used as computed fields can also accept other arguments other than the mandatory table row argument. Values for these extra arguments can be passed as arguments to the computed field in the GraphQL API.

Computed field types

Based on the SQL function’s return type, we can define two types of computed fields:

1. Scalar computed fields

Computed fields whose associated SQL function returns a base type like Integer, Boolean, Geography etc. are scalar computed fields.

Example:

Let’s say we have the following schema:

author(id integer, first_name text, last_name text)

Define an SQL function called author_full_name:

CREATE FUNCTION author_full_name(author_row author)
RETURNS TEXT AS $$
  SELECT author_row.first_name || ' ' || author_row.last_name
$$ LANGUAGE sql STABLE;

Add a computed field called full_name to the author table using the SQL function above.

Query data from the author table:

query {
  author {
    id
    first_name
    last_name
    full_name
  }
}
query { author { id first_name last_name full_name } }
{ "data": { "author": [ { "id": 1, "first_name": "Chris", "last_name": "Raichael", "full_name": "Chris Raichael" } ] } }

2. Table computed fields

Computed fields whose associated SQL function returns SETOF <table-name> are table computed fields. The return table must be tracked to define such a computed field.

Example:

Let’s say we have the following schema:

author(id integer, first_name text, last_name text)

article(id integer, title text, content text, author_id integer)

Now we can define a table relationship on the author table to fetch authors along with their articles.

We can make use of computed fields to fetch the author’s articles with a search parameter.

Define an SQL function called filter_author_articles:

CREATE FUNCTION filter_author_articles(author_row author, search text)
RETURNS SETOF article AS $$
  SELECT *
  FROM article
  WHERE
    ( title ilike ('%' || search || '%')
      OR content ilike ('%' || search || '%')
    ) AND author_id = author_row.id
$$ LANGUAGE sql STABLE;

Add a computed field called filtered_articles to the author table using the SQL function above.

Query data from the author table:

query {
  author {
    id
    first_name
    last_name
    filtered_articles(args: {search: "Hasura"}){
      id
      title
      content
    }
  }
}
query { author { id first_name last_name filtered_articles(args: {search: "Hasura"}){ id title content } } }
{ "data": { "author": [ { "id": 1, "first_name": "Chris", "last_name": "Raichael", "filtered_articles": [ { "id": 1, "title": "Computed fields in Hasura", "content": "lorem ipsum dolor sit amet" } ] } ] } }

Adding a computed field to a table

Head to the Modify tab of the table and click on the Add button in the Computed fields section:

Supported from

Console support is available in v1.1.0 and above

You can add a computed field in the tables.yaml file inside the metadata directory:

 - table:
     schema: public
     name: author
   computed_fields:
   - name: full_name
     definition:
       function:
         schema: public
         name: author_full_name
       table_argument: null
     comment: ""

Apply the metadata by running:

hasura metadata apply

A computed field can be added to a table using the add_computed_field metadata API:

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

{
  "type": "add_computed_field",
  "args": {
    "table": {
      "name": "author",
      "schema": "public"
    },
    "name": "full_name",
    "definition": {
      "function": {
        "name": "author_full_name",
        "schema": "public"
      },
      "table_argument": "author_row"
    }
  }
}

Computed fields permissions

Access control to computed fields depends on the type of computed field.

  • For scalar computed fields, permissions are managed similar to the columns permissions of the table.
  • For table computed fields, the permissions set on the return table are respected.

Accessing Hasura session variables in computed fields

It can be useful to have access to the session variable from the SQL function defining a computed field. For instance, suppose we want to record which users have liked which articles. We can do so using a table article_likes that specifies a many-to-many relationship between article and user. In such a case it can be useful to know if the current user has liked a specific article, and this information can be exposed as a Boolean computed field on article.

Create a function with an argument for session variables and add it with the add_computed_field API with the session_argument key set. The session argument is a JSON object where keys are session variable names (in lower case) and values are strings. Use the ->> JSON operator to fetch the value of a session variable as shown in the following example.

-- 'hasura_session' will be the session argument
CREATE OR REPLACE FUNCTION article_liked_by_user(article_row article, hasura_session json)
RETURNS boolean AS $$
SELECT EXISTS (
    SELECT 1
    FROM article_likes A
    WHERE A.user_id = hasura_session ->> 'x-hasura-user-id' AND A.article_id = article_row.id
);
$$ LANGUAGE sql STABLE;
POST /v1/query HTTP/1.1
Content-Type: application/json
X-Hasura-Role: admin

{
    "type":"add_computed_field",
    "args":{
        "table":{
            "name":"article",
            "schema":"public"
        },
        "name":"liked_by_user",
        "definition":{
            "function":{
                "name":"article_liked_by_user",
                "schema":"public"
            },
            "table_argument":"article_row",
            "session_argument":"hasura_session"
        }
    }
}
query {
  article(where: {id: {_eq: 3}}) {
    id
    liked_by_user
  }
}
query { article(where: {id: {_eq: 3}}) { id liked_by_user } }
{ "data": { "article": [ { "id": "3", "liked_by_user": true } ] } }

Note

The specified session argument is not included in the argument options of the computed field in the GraphQL schema.

Supported from

This feature is available in v1.3.0-beta.1 and above

Computed fields vs. Postgres generated columns

Postgres, from version 12, is introducing Generated Columns. The value of generated columns is also computed from other columns of a table. Postgres’ generated columns come with their own limitations. Hasura’s computed fields are defined via an SQL function, which allows users to define any complex business logic in a function. Generated columns will go together with computed fields where Hasura treats generated columns as normal Postgres columns.