Defining the Schema

The schema.graphql file serves as a representation of your application's data model. It defines entity types that directly correspond to database tables, and the event handlers you create are responsible for creating and updating records within those tables. Additionally, the GraphQL API is automatically generated based on the entity types specified in the schema.graphql file, to allow access to the indexed data.

Entity types are identified with the directive within the schema.graphql file.

Example schema from the Greeter template:

type User {
  id: ID!
  greetings: [String!]!
  latestGreeting: String!
  numberOfGreetings: Int!
}

Every entity type must include an id field that is of type ID!, String!, Int!, Bytes!, or BigInt!. The id field serves as a unique identifier for each instance of the entity.

Enums

The schema file also supports the use of enum types. An example of an enum definition and usage within the schema is shown below:

enum AccountType {
  ADMIN
  USER
}
type User {
  id: ID!
  balance: Int!
  accountType: AccountType!
}

Enum types are generated as string union types for TypeScript and JavaScript and as polymorphic variants for ReScript. Therefore to set an enum field in an entity in TypeScript and JavaScript, the string of the enum value is used:

import { AccountType } from "../generated/src/Enums.gen";

let accountType: AccountType = "USER";

let user = {
  id: event.params.id,
  balance: event.params.balance,
  accountType,
};

For ReScript, we use the polymorphic variant

let accountType: Enums.accountType = #USER;

let user: Types.userEntity = {
  id: event.params.id,
  balance: event.params.balance,
  accountType
};

Scalar Types

In GraphQL, scalars represent fundamental data types such as strings and numbers. Each GraphQL scalar is mapped to a corresponding JavaScript, TypeScript, or ReScript type, which is used in event handler code, depending on the language chosen. The following table provides an overview of the available scalar types, along with their associated JavaScript, TypeScript, and ReScript types:

Name	Description	JavaScript/TypeScript Type	ReScript Type
ID	A unique identifier field	string	string
String	A UTF-8 character sequence	string	string
Int	A signed 32-bit integer	number	int
Float	A signed floating-point value	number	float
Boolean	Represents a true or false value	boolean	bool
Bytes	A UTF-8 character sequence with a 0x prefix	string	string
BigInt	A signed integer (equivalent to solidity int256)	bigint	bigint
BigDecimal	An arbitrary size floating point number	BigDecimal (imported from "generated")	BigDecimal.t
Timestamp	Timestamp with time zone	Date	Js.Date.t

You can find out more on GraphQL here.

Once you have set up your config and schema file you can run envio codegen to generate the functions that you will use in your handlers.

envio codegen

Defining One-to-Many Relationships

type NftCollection {
  id: ID!
  contractAddress: Bytes!
  name: String!
  symbol: String!
  maxSupply: BigInt!
  currentSupply: Int!
  tokens: [Token!]! @derivedFrom(field: "collection")
}

type Token {
  id: ID!
  tokenId: BigInt!
  collection: NftCollection!
  owner: User!
}

Assume that each NftCollection can have multiple Token objects. This is represented by the [Token!] in NftCollection definition, where the field's type is set to another entity type.

When you create a Token entity, the value of the collection field is set to the id of its associated NftCollection entity.

Note that in the NftCollection schema, the tokens field can't be directly accessed or modified. Fields marked with the @derivedFrom directive function are virtual fields and are only accessible when executing GraphQL API queries. This is commonly known as reverse lookup, as the relationship is established on the "many" end of the association.

Adding Indexes to Fields

Add an index to a field to improve read performance and enable field queries in your loaders.

Indices will automatically be added to all entity id fields and all fields referenced using @derivedFrom directive.

Example

type Token {
  id: ID!
  tokenId: BigInt!
  collection: NftCollection!
  owner: User! @index
}

Advanced: @config Directive for Decimal Precision of BigInt and BigDecimal Fields

When working with large integers or high-precision decimal numbers in your application, you might need to customize the precision and scale of your BigInt and BigDecimal fields. This ensures that your database stores these numbers accurately according to your specific requirements. If you know your numbers will not be too big, you can also optimize the database by not over-allocating on the precision.

Using the @config Directive

The @config directive allows you to specify custom configurations for fields. For BigInt and BigDecimal types, you can define precision and scale parameters.

Syntax for BigInt:

fieldName: BigInt @config(precision: <number_of_digits>)

Syntax for BigDecimal:

fieldName: BigDecimal @config(precision: <total_digits>, scale: <decimal_digits>)

Example:

type Entity {
    id: ID!
    amount: BigInt @config(precision: 76)
    price: BigDecimal @config(precision: 10, scale: 2)
}

In this example:

• The amount field is a BigInt with up to 76 digits.
• The price field is a BigDecimal with a total of 10 digits and 2 decimal places.

Click to expand a complete example of @config directive usage

Complete Example Usage

type Entity {
    id: ID!
    exampleBigInt: BigInt @config(precision: 76)
    exampleBigIntRequired: BigInt! @config(precision: 77)
    exampleBigIntArray: [BigInt!] @config(precision: 78)
    exampleBigIntArrayRequired: [BigInt!]! @config(precision: 79)
    exampleBigDecimal: BigDecimal @config(precision: 80, scale: 5)
    exampleBigDecimalRequired: BigDecimal! @config(precision: 81, scale: 5)
    exampleBigDecimalArray: [BigDecimal!] @config(precision: 82, scale: 5)
    exampleBigDecimalArrayRequired: [BigDecimal!]! @config(precision: 83, scale: 5)
}

Postgres Precision and Scale Details

Click to expand Postgres precision and scale details

In PostgreSQL, the NUMERIC data type is used to store exact numeric values with user-defined precision and scale. Understanding how these work is crucial when customizing your numeric fields.

• Precision: Total number of significant digits in the number (both to the left and right of the decimal point).
• Scale: Number of digits after the decimal point.

Examples:

• A number 12345.678 has a precision of 8 and a scale of 3.
• With a NUMERIC(10, 2) data type, you can store numbers up to 99999999.99.

Key Points:

• Precision and Scale Limits:

  • The maximum number of digits to the left of the decimal point is precision - scale.
  • The total number of digits cannot exceed the specified precision.

• Rounding and Truncation:

  • If you insert a number with more decimal places than the specified scale, PostgreSQL will round it.
  • If the integer part exceeds the allowed digits (precision - scale), PostgreSQL will raise an error.

• Storage Size:

  • The storage size of a NUMERIC field depends on its declared precision.

By customizing precision and scale in your schema, you directly influence how PostgreSQL stores and validates your numeric data, ensuring data integrity and optimal storage usage.

Additional Resources:

• PostgreSQL Numeric Types Documentation

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Other Design Tips

• Use lowercase for the first letter of field names (i.e. latestGreeting and numberOfGreetings) inside entities.

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