azurerm_cosmosdb_mongo_collection - fix resource creation and update by adjusting '_id' index specification

[jwalaszek]

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Description

Underlying Azure REST API does not accept unique property for _id index specification. This index is always unique by design and API call fails when this property is set to either true or false. This causes issues when unique field is set in Terraform configuration file - both true and false values result in a failure during azurerm_cosmosdb_mongo_collection resource creation.

Existing behavior during resource creation:

• If _id index's unique property is set to true or false, then resource creation fails with The field 'unique' is not valid for _id index specification. message. This is exactly this issue: Mongo collection not supporting unique field on _id index #24853
• If _id index's unique property is not set, then resource is created successfully (with unique _id index, despite the default for this unique property being false)

Existing behavior during resource update:

• If nothing has changed (unique is still not set), terraform plan shows that unique property is currently true but needs to be changed to false (as it's the default value). Running terraform apply is successful, but it does not change anything in Azure and does not fix invalid terraform state, resulting in repeating this behavior each time terraform plan is executed, as described in landintrees's comment and BosBer's comment
• If _id index's unique property is changed to true, then terraform plan outputs no changes to be made.
• If _id index's unique property is changed to false, then terraform apply fails with The field 'unique' is not valid for _id index specification. message.

Expected behavior during resource creation:

• If _id index's unique property is not set or set to false, then resource creation fails with index with '_id' key and 'unique' property set to true is required message.
• If _id index's unique property is set to true, then resource is created successfully.

Expected behavior during resource update:

• If _id index's unique property is not set or set to false, then terraform plan fails with index with '_id' key and 'unique' property set to true is required message.
• If _id index's unique property is still set to true, then terraform plan outputs no changes to be made.

PR Checklist

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Changes to existing Resource / Data Source

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Testing

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Change Log

Below please provide what should go into the changelog (if anything) conforming to the Changelog Format documented here.

• azurerm_resource - support for the thing1 property [GH-00000]

This is a (please select all that apply):

• Bug Fix
• New Feature (ie adding a service, resource, or data source)
• Enhancement
• Breaking Change

Related Issue(s)

Fixes #24853

Note

If this PR changes meaningfully during the course of review please update the title and description as required.

[rcskosir]

Hi @jwalaszek 👋,
Thank you for taking the time to open this PR and contributing to the repository. Apologies for the delay in getting this PR reviewed.
Mongo has made changes to id that makes this change require extra investigation on our part and we want to ensure that we investigate this change thoroughly.

Please bear with us as we work through this, and we’ll get back to you as soon as possible with more feedback.