How to Copy and Sync a MongoDB Collection to PostgreSQL

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Sync a MongoDB collection to PostgreSQL with VisuaLeaf.

I tested a MongoDB-to-PostgreSQL sync with a visits collection. At first, I thought the hard part would be the sync itself. It was not. The harder part was making PostgreSQL understand the MongoDB document shape. MongoDB lets one document contain strings, dates, nested objects, arrays, and an _id. PostgreSQL wants fixed columns, clear data types, and a primary key. So before running the sync, I had to decide what the PostgreSQL table should look like. For this test, I used VisuaLeaf Mongo Sync. I used it because I wanted to set up the workflow visually: source collection, target PostgreSQL table, field mapping, and sync status. I still had to create the PostgreSQL table and check the mapping carefully, but I did not have to write a custom sync script. Source Collection The source was: database_compare_demo.visits Each MongoDB document had simple fields like: _id visitId status visitDate visitReason It also had nested fields: patient doctor symptoms prescriptions invoice labResults visitDetails vitals clinic MongoDB visits collection with nested patient, doctor, lab results, and vitals data.This is where the mapping matters. If you try to force every nested MongoDB field into separate SQL columns too early, the setup gets messy fast. For this test, I used a PostgreSQL landing table. PostgreSQL Table I created one table: CREATE TABLE public.mongo_visits_sync ( mongo_id TEXT PRIMARY KEY, "visit_id" TEXT, status TEXT, "visit_date" TIMESTAMPTZ, "visit_reason" TEXT,

patient JSONB, doctor JSONB, symptoms JSONB, prescriptions JSONB, invoice JSONB, "lab_results" JSONB, "visit_details" JSONB, vitals JSONB, clinic JSONB ); I kept the simple fields as normal columns. I stored the nested MongoDB objects and arrays as JSONB. This was easier to test, and it kept the original document structure instead of flattening everything too early. Why mongo_id Is Important The PostgreSQL table needs a primary key. In this setup, MongoDB _id maps to PostgreSQL mongo_id. _id -> mongo_id Without this, the sync cannot safely update existing rows. One error I hit was: SQL target has no key columns — cannot upsert idempotently That means PostgreSQL did not have a usable key for the sync job. The fix was to create mongo_id as the primary key and map MongoDB _id to it. Field Mapping and Data Types In VisuaLeaf, the mapping shows the MongoDB fields with simple source types like String, Object, and Array. That matters because not every MongoDB field should become a normal text column in PostgreSQL. Creating a new transformation mapping before generating the MongoDB to PostgreSQL fields.For this test, I used this logic:

MongoDB field Type shown in VisuaLeaf PostgreSQL column PostgreSQL type

_id String mongo_id TEXT

visitId String visit_id TEXT

status String status TEXT

visitDate String / Date visit_date TIMESTAMPTZ

visitReason String visit_reason TEXT

patient Object patient JSONB

doctor Object doctor JSONB

symptoms Array symptoms JSONB

prescriptions Array prescriptions JSONB

invoice Object invoice JSONB

labResults Array / Object lab_results JSONB

visitDetails Object visit_details JSONB

vitals Object vitals JSONB

clinic Object clinic JSONB

Field mapping from MongoDB fields to PostgreSQL columns, including objects and arrays.The simple string fields went into normal PostgreSQL text...

How to Copy and Sync a MongoDB Collection to PostgreSQL

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