Implement "migrationActions" to allow migration of data / data conversions between syncActions updates

该 issue 没有指明仓库文件、测试或现有的迁移入口。首先定位 syncActions 的实现和相关 issue #355，然后确定当前如何处理自定义字段和产品属性。完成的要求是具备已达成一致的迁移设计、转换行为、清理操作，以及对受支持资源类型的覆盖。

Hi,
wouldn't it be awesome if we could update custom types or product types field definitions with a migration strategy to prevent data loss? As far I know, It doesn't exist any assistant in migrating, for example, a ProductVariant attribute between different field types.

Scope:

It will affect custom field definitions (Types, ProductTypes). That means migration support for product attributes, custom attributes of all supported resources. For the base fields, it's not necessary since the type will never change.

Use cases:

• A user might want to change the field String to LocalizedString
• A user might want to change the field String to Number
• A user might want to change the field String to SetOfStrings

With migrationActions I think about a companion to control how the old data is preserved.

That feature request comes from the daily practice experience with CT. Our product is very new and the model is constantly evolving.

I think this feature would bring a lot of value to the platform because it allows writing migrations scripts in a convenient and safe way.

That migration tool can be implemented in a very generic way so we don't need to respect every resource type individually.

It would require that "syncActions" can handle changes in custom attributes. I have seen that the current implementation respects only base fields or the implementation is just not complete.

Out of scope:

Updates in base fields are covered by sync-actions whereby the implementation doesn't cover all cases e.g there is no support for custom fields in types or product-types. That half complete implementation is really hard to track.

It's also really hard to find out what sync really means in the context of the "resourceType". As a customer, I would expect that sync checks EVERYTHING or at least the documentation should clarify the exact behavior.

Related issues:

https://github.com/commercetools/nodejs/issues/355

Some code to illustrate the migration approach:

1. Determine attribute changes

import { typeConversionMap } from '@commercetools/migration-actions';

const updateActions = productTypeSync.buildActions(next, previous, { preservePreviousFields: true });

// for example the attribute type has changed from "String" to "Number"
// we will create a new field to hold the new value + saving type and name differences in the attribute name

updateActions = [
  {
    action: 'addAttributeDefinition',
    attribute: {
      name: 'previous-attribute-name__string-number__next-attribute-name',
      type: { name: 'Number' }
    }
  }
];

2. Apply changes and create temporary fields to hold the new value + conversion information

client.execute(updates);

3. Fetch records which should be migrated

// for example
const productA = {
  attributes: [
    { name: 'previous-attribute-name__string-number__next-attribute-name', value: null },
    { name: 'previous-attribute-name', value: '2' }
  ]
};

4. We will create actions to migrate all fields

With the convention "old__string-number__new" or even "old__oldType-newType__old" we know how to convert that field.

const typeConversionHelper = {
  [[typeConversionMap.String][typeConversionMap.Number]]: old => parseInt(old),
  [[typeConversionMap.String][typeConversionMap.ListString]]: old => [old.toString()],
  [[typeConversionMap.String][typeConversionMap.LocalizedString]]: old => { en: old }
};
const dataActions = migrationSync.buildActions([productA], { conversionHelper });

dataActions = {
  url: 'https://api/products/....',
  body: {
    actions: [
      {
        action: 'setAttribute',
        name: 'previous-attribute-name__string-number__next-attribute-name',
        value: 2
      }
    ]
  }
};

5. Determine changes based on the naming convention and remove old fields and rename temporary fields

const dataActions = migrationSync.migrationCleanActions([productA]);

dataActions = [
  {
    action: 'removeAttributeDefinition',
    name: 'previous-attribute-name'
  },
  {
    action: 'changeAttributeName',
    attributeName: 'previous-attribute-name__string-number__next-attribute-name',
    newAttributeName: 'next-attribute-name'
  }
];