Migrating synced Realm Schema

MongoDB Atlas App Services & Realm Atlas Device Sync
, ,
1

Creating a partner collection to production is performed with
Aggregation Pipeline with `
db.Accounts.aggregate([
{ $match: {} },
{ $out: “AccountsV1” },
])
Accounts collection uses relation ships. After aggreagte the new collection can’t be use in Swift because the relationsship seems to have an issue:
‘AccountsV1.transactions’ has been changed from
‘array’ to ‘array’
Normally the aggregation should handle the relation setup as well, but currently only manual relation setup seems to work. Anyone with the same issue? Thx

2

The issue you’re facing seems to stem from the fact that when you use the $out stage in MongoDB’s aggregation pipeline to create a new collection (AccountsV1), the relationships or data structures in the new collection (AccountsV1.transactions) may not perfectly match the original Accounts collection. This can result in issues when trying to use the new collection in your Swift application, especially if the Swift code relies on specific data structures or relationships.

To solve the issue:

  • Verify the data structure post-aggregation.
  • Explicitly handle nested fields in your aggregation pipeline.
  • Validate schemas before and after the aggregation.
  • Adjust your Swift code if necessary to accommodate any changes.

Here’s a breakdown of potential causes and some strategies to address this:

  1. Aggregation Pipeline and Relationships
    The aggregation pipeline, particularly the $out stage, is designed to transform documents and output the results into a new collection. However, it does not inherently preserve or translate complex relationships, especially those involving nested arrays or embedded documents, unless explicitly handled in the pipeline.

    Potential Problem:
    If your Accounts.transactions field is an array of embedded documents or a nested structure, and the aggregation pipeline does not explicitly maintain the schema or structure, the new collection may have a different schema.

  2. Manual Relationship Setup
    If the relationship is not automatically preserved, you might need to manually ensure that the aggregation pipeline is correctly handling nested fields and relationships.

    How to fix this:

    • Explicit Mapping: Explicitly map and transform the nested fields within the pipeline to ensure they match the expected structure. For example:
 db.Accounts.aggregate([
       { $match: {} },
       { 
         $project: {
           _id: 1,
           transactions: 1,
           // Map any other necessary fields here
         }
       },
       { $out: "AccountsV1" }
     ])

 This ensures that the `transactions` array (or any other nested fields) is explicitly included in the new collection.
  1. Schema Validation
    MongoDB is schema-less, which means that changes in the structure, even subtle ones, might not be caught until you try to use the data. To avoid this, you could use schema validation or run tests to ensure the schema remains consistent after the aggregation.

A set of solutions to this:

  • Use Mongoose or a similar ORM: If you’re using Mongoose (or similar) in Node.js, or an ORM in Swift, make sure that the schema validation is consistent across both the original and new collections.
  • Schema Comparison: Before and after running the aggregation, compare the schemas of Accounts and AccountsV1. You can use tools or write scripts to check if the structure matches.
  1. Debugging in Swift
    If the issue persists in Swift, consider logging or debugging the data structure of AccountsV1.transactions to ensure that the structure is exactly what your Swift models expect.

** How:**

  • Print or log the structure: Before processing AccountsV1, log or print the transactions field to ensure it matches your model expectations.
  • Model Adaptation: If necessary, adapt your Swift models to handle any slight changes or discrepancies in the new collection’s structure.
  1. Recreating Relationships
    If the relationships are complex and involve lookups or joins, consider rebuilding the relationships within the aggregation pipeline.

The solution

  • Use $lookup and $unwind: To reconstruct relationships, use the $lookup and $unwind stages within your aggregation pipeline to ensure that related documents are properly linked.
 db.Accounts.aggregate([
       { $match: {} },
       {
         $lookup: {
           from: "RelatedCollection",
           localField: "transactions.relatedId",
           foreignField: "_id",
           as: "relatedData"
         }
       },
       { $unwind: "$relatedData" },
       { $out: "AccountsV1" }
     ])
  1. Versioning Collections
    Consider versioning your collections (AccountsV1, AccountsV2, etc.) and maintaining strict checks when migrating data to ensure that every version matches the expected structure.

If you continue to face issues, sharing specific examples of your data structure and aggregation pipeline could help the community provide more targeted advice.