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Version: Old ArrowSquid docs

Union type resolution

Use cases for Union types have been discussed in the schema reference. Here, we discuss how to query union types.

Let's take this modified schema from the Substrate tutorial:

schema.graphql
type Account @entity {
id: ID! #Account address
events: [Event]
}

type WorkReport {
id: ID! #event id
addedFiles: [[String]]
deletedFiles: [[String]]
extrinsicId: String
blockHash: String!
}

type JoinGroup {
id: ID!
owner: String!
extrinsicId: String
blockHash: String!
}

type StorageOrder {
id: ID!
fileCid: String!
extrinsicId: String
blockHash: String!
}

union Event = WorkReport | JoinGroup | StorageOrder

Here, an Event will have different fields depending on the underlying type. This query demonstrates how to request different fields for each of these types:

query MyQuery {
accounts {
events {
__typename
... on WorkReport {
id
blockHash
extrinsicId
deletedFiles
}
... on JoinGroup {
id
blockHash
extrinsicId
}
... on StorageOrder {
id
blockHash
extrinsicId
}
}
id
}
}

The special __typename field allows users to discern the returned object type without relying on comparing the sets of regular fields. For example, in the output of the query above JoinGroup and StorageOrder events can only be distingushed by looking at the __typename field. Here is a possible output to illustrate:

{
"data": {
"accounts": [
{
"events": [
{
"__typename": "WorkReport",
"id": "0000584321-000001-01cdb",
"blockHash": "0x01cdb3cb6fa00f62fd20220104f1d740a53518b63517419da8a89325d065562b",
"extrinsicId": "0000584321-000001-01cdb",
"deletedFiles": []
}
],
"id": "cTKmzHG3RHa1yhujyZpPnNL17p8a48Av3JFwDjpttLcxeSo26"
},
{
"events": [
{
"__typename": "JoinGroup",
"id": "0000584598-000010-d06ec",
"blockHash": "0xd06ec6716e96108e24987ef03d23c857ef3b467dd057d7a32c4e123fe5a8df36",
"extrinsicId": "0000584598-000004-d06ec"
}
],
"id": "cTKqevWRdvbNNAQ3hLxhsNYhQ8pf5YGkYnnVjgjLNiVr4kd7a"
},
{
"events": [
{
"__typename": "StorageOrder",
"id": "0000584627-000013-1fa19",
"blockHash": "0x1fa19ae98731afad853ffd491fcbc0c3dcda6b8b7f5a2d56ac6c4c1eb9e4f95e",
"extrinsicId": "0000584627-000005-1fa19"
}
],
"id": "cTGYF8jvcpnRmgNopqT4nVs5rWHEviAAdRdfNrZE8NFz2Av7B"
}
]
}
}