bc-skills/creating-bc-custom-api

Create custom API pages (CRUD) and API queries (read-only joins) in AL

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name:: creating-bc-custom-api
description:: Creates custom API pages (CRUD) and API queries (read-only joins) in AL for Business Central. Use when exposing custom tables, adding business logic to APIs, creating reporting endpoints, or joining multiple tables for external consumption.
license:: MIT
metadata:: {"version":"1.0.0"}

Skill: Creating BC Custom APIs

Name: bc-skills/creating-bc-custom-api
Author: bc-skills

Key Concept

Type	Object	Operations	Use Case
API Page	`PageType = API`	CRUD (Read-Write)	Expose single table
API Query	`QueryType = API`	Read-only	Join multiple tables

For consuming Microsoft standard APIs, use al-standard-api skill instead.

Validation Gates

After Step 2: API page compiles, GET returns 200 with fields
After Step 3: Subpage works, navigation property expands correctly
Final: All CRUD operations work (GET 200, POST 201, PATCH 200, DELETE 204)

Note: OData UI endpoints deprecated BC30 (2027). Always use API pages.

Placeholder Reference

Placeholder	Description	Example
`<ID>`	Object ID from app.json idRanges	`50100`
`<PREFIX>`	Company prefix from app.json	`SL`, `ABC`
`<SourceTable>`	Source table name	`"Rental Machine"`
`<entityName>`	Singular entity name (camelCase)	`rentalMachine`
`<entitySetName>`	Plural entity name (camelCase)	`rentalMachines`
`<publisher>`	APIPublisher value	`contoso`
`<group>`	APIGroup value	`rental`
`<KeyField>`	Primary key field	`"No."`
`<Field1>`, `<Field2>`	Table fields to expose	`"Description"`, `"Status"`

Procedure

Step 1: Decide API Type

Use API Page when:

Need CRUD operations (Create, Read, Update, Delete)
Exposing a single table
Need to execute business logic on insert/modify

Use API Query when:

Read-only access sufficient
Need to join multiple tables
Reporting/analytics endpoint

Step 2: Create API Page (CRUD)

page <ID> "<PREFIX> API <EntityName>"
{
    APIGroup = '<group>';
    APIPublisher = '<publisher>';
    APIVersion = 'v2.0';
    EntityName = '<entityName>';
    EntitySetName = '<entitySetName>';
    EntityCaption = '<Entity Display Name>';
    EntitySetCaption = '<Entities Display Name>';
    PageType = API;
    SourceTable = <SourceTable>;
    DelayedInsert = true;
    ODataKeyFields = SystemId;

    layout
    {
        area(Content)
        {
            repeater(General)
            {
                field(id; Rec.SystemId)
                {
                    Caption = 'ID';
                    Editable = false;
                }
                field(<fieldApiName1>; Rec.<KeyField>)
                {
                    Caption = '<Field1 Caption>';
                }
                field(<fieldApiName2>; Rec.<Field1>)
                {
                    Caption = '<Field2 Caption>';
                }
                field(<fieldApiName3>; Rec.<Field2>)
                {
                    Caption = '<Field3 Caption>';
                    Editable = false;  // Calculated fields
                }
                field(lastModifiedDateTime; Rec.SystemModifiedAt)
                {
                    Caption = 'Last Modified';
                    Editable = false;
                }
            }
        }
    }

    trigger OnInsertRecord(BelowxRec: Boolean): Boolean
    begin
        // Optional: Set default values or execute business logic
        Rec.Insert(true);
        exit(false);  // Return false to prevent double insert
    end;
}

Step 3: Add Navigation Property (Lines/Details)

For header-line relationships, add a part to the parent API page:

layout
{
    area(Content)
    {
        repeater(General)
        {
            // ... header fields ...
        }
        part(<entityName>Lines; "<PREFIX> API <EntityName> Lines")
        {
            EntityName = '<entityName>Line';
            EntitySetName = '<entityName>Lines';
            SubPageLink = <ParentKeyField> = field(<KeyField>);
        }
    }
}

Subpage (Lines):

page <ID+1> "<PREFIX> API <EntityName> Lines"
{
    APIGroup = '<group>';
    APIPublisher = '<publisher>';
    APIVersion = 'v2.0';
    EntityName = '<entityName>Line';
    EntitySetName = '<entityName>Lines';
    PageType = API;
    SourceTable = <LineSourceTable>;
    DelayedInsert = true;
    ODataKeyFields = SystemId;

    layout
    {
        area(Content)
        {
            repeater(General)
            {
                field(id; Rec.SystemId) { Editable = false; }
                field(<parentKeyApiName>; Rec.<ParentKeyField>) { }
                field(lineNo; Rec."Line No.") { }
                // ... other line fields ...
            }
        }
    }
}

Step 4: Create API Query (Read-Only Joins)

query <ID> "<PREFIX> API <QueryName>"
{
    QueryType = API;
    APIPublisher = '<publisher>';
    APIGroup = '<group>';
    APIVersion = 'v2.0';
    EntityName = '<entityName>';
    EntitySetName = '<entitySetName>';

    elements
    {
        dataitem(<HeaderAlias>; <HeaderTable>)
        {
            column(id; SystemId) { }
            column(<headerField1>; <KeyField>) { }
            column(<headerField2>; <Field1>) { }

            dataitem(<LineAlias>; <LineTable>)
            {
                DataItemLink = <LinkField> = <HeaderAlias>.<KeyField>;
                SqlJoinType = InnerJoin;  // or LeftOuterJoin

                column(lineNo; "Line No.") { }
                column(<lineField1>; <LineField1>) { }
                column(<lineField2>; <LineField2>) { }
            }
        }
    }
}

Required Properties

Property	Required	Description
`APIPublisher`	Yes	Your company/publisher name
`APIGroup`	Yes	Logical grouping
`APIVersion`	Yes	`v2.0` recommended, `beta` for development
`EntityName`	Yes	Singular, camelCase
`EntitySetName`	Yes	Plural, camelCase
`ODataKeyFields`	Yes	Always use `SystemId`
`PageType`/`QueryType`	Yes	`API`
`DelayedInsert`	Recommended	`true` for API pages

API URL Structure

Custom API endpoint:

https://api.businesscentral.dynamics.com/v2.0/<TenantID>/<Environment>/api/<publisher>/<group>/<version>/companies(<CompanyID>)/<entitySetName>

Example:

https://api.businesscentral.dynamics.com/v2.0/contoso.com/Production/api/contoso/rental/v2.0/companies(xxx)/rentalMachines

Schema Version and OData Features (BC v24+)

Enum Values (schemaversion 2.0)

Starting BC v24, enum values are returned as XML-encoded names:

Return Order → Return_x0020_Order

Add ?$schemaversion=1.0 for old behavior.

IN Operator (schemaversion 2.1)

?$schemaversion=2.1&$filter=status in ('Open', 'Released')

Without it → BadRequest_MethodNotImplemented.

Filter inside $expand

?$expand=lines($filter=type eq 'Item')

Parentheses required around nested options.

Multi-level Expand

?$expand=lines($expand=item($expand=category))

Best Practices

Always use ODataKeyFields = SystemId - stable across renames
Use EntityCaption/EntitySetCaption - for localization at /entityDefinitions
camelCase for API names - rentalMachine not RentalMachine
Alphanumeric only - no special characters in field names
Version your APIs - v1.0, v2.0, or beta
Include SystemModifiedAt - enables delta sync with $filter
DelayedInsert = true - prevents partial record creation

CRUD Control

Make API read-only:

page <ID> "<PREFIX> API Read Only"
{
    PageType = API;
    InsertAllowed = false;
    ModifyAllowed = false;
    DeleteAllowed = false;
}

Troubleshooting

Error	Cause	Fix
400	Invalid field name	Check camelCase, no spaces
404	Entity not found	Verify `ODataKeyFields = SystemId`, entity names
500	Server error	Check `OnInsertRecord` trigger, BC event log
API not visible	Not published	Restart service tier, check compilation

Feedback loop: Fix issue → Recompile → Test GET first → Then test POST/PATCH/DELETE.

References

See references/ folder for:

api-page-template.al - Complete AL template for API Page (CRUD), Subpage (Lines), and Read-Only API
api-query-template.al - Complete AL template for API Query with join examples

External Documentation

SKILL.md

tile.json