Why is Postgres jsonb Insertion Failing When Passing a JSON Array Instead of an Object?

Below is an explanation of the issue and some guidance on how to resolve it.

What's Happening

The error

Invalid input for ‘mentions’ [item 0]
‘mentions’ expects a object but we got array

indicates that when you map your response JSON to the Postgres node, the field (in this case, "mentions" or one of your array fields) is expected to be a JSON object (key/value pairs) but an array is being provided instead. Even though Postgres itself can store arrays in a jsonb column, n8n’s mapping and type conversion logic expects the JSON format to match the column’s mapping.

Why It Might Occur

• Mapping Expectations: The Postgres node in n8n is expecting an object to be mapped for the jsonb type. When you pass an array (even if it’s an empty array) directly from your response JSON, it does not match that expected format.
• Stringifying Won't Help: Wrapping the array in JSON.stringify() will simply convert it into a string. This string may not be automatically cast back to a jsonb structure, causing the conflict.
• Column DataType: Changing the Postgres column to jsonb[] doesn’t work because the node’s internal validation still checks the input data to ensure it’s an object formatted as a valid JSON object.

How to Fix It

1. Convert the Array to an Object:
   One workaround is to wrap your array within an object. For example, instead of storing:

   JSONCopied
   { "mentions": [] }

   change the data to something like:

   JSONCopied
   { "mentions": { "data": [] } }

   This way, when the Postgres node validates the input, it receives an object. You may later extract the array using your SQL or application logic.

2. Adjust the Node Mapping:
   If your intention is to store an array as jsonb, look into the mapping mode available in the Postgres insert node. For instance, using "Map Automatically" mode may treat the incoming field as a whole. However, if the validation error still occurs, explicitly wrapping the array in an object before the node is executed is recommended.

3. Check Your Expressions:
   If you’re using expressions to transform your data before inserting, double check that your expression returns an object rather than an array. For example, instead of:

   JSCopied
   {{ $json.mentions }}

   you might want to do:

   JSCopied
   {{ { "data": $json.mentions } }}

   so that the mapping returns an object.

Summary

The error is due to the type mismatch where the Postgres node expects an object for a jsonb field but is receiving an array. To work around this, transform your array into an object (for instance, by wrapping it in a property such as “data”). This should align the input with the expected schema and allow the Postgres insert to proceed without error.

If you have further details (such as intermediate steps or additional mappings), those might help fine-tune the required transformation.