Downgrade 3.4 Replica Set to 3.2

1. Downgrade Feature Compatibility Version

1. Connect a mongo shell to the primary.

2. Downgrade the featureCompatibilityVersion to "3.2".

   db.adminCommand({setFeatureCompatibilityVersion: "3.2"})

   This command must perform writes to an internal system collection. If for any reason the command does not complete successfully, you can safely retry the command on the primary as the operation is idempotent.

2. Remove Views

If you have defined any views, drop the views before downgrading MongoDB 3.4 to 3.2.

1. Connect a mongo shell to the primary.

2. To find views, you can run the following in the mongo shell:

   db.adminCommand("listDatabases").databases.forEach(function(d){
      let mdb = db.getSiblingDB(d.name);
      mdb.getCollectionInfos({type: "view"}).forEach(function(c){
         print(mdb[c.name]);
      });
   });

   In each database that contains views, drop the system.views collection to drop all views in that database.

   If running with access control, you must have privileges to drop the system.views collection for the database. See Create a Role to Drop system.views Collection across Databases.

3. Remove Collation Option from Collections and Indexes

If you have defined any non-"simple" collation for a collection or an index, remove the collection or index before downgrading MongoDB 3.4 to 3.2.

1. Connect a mongo shell to the primary.

2. To find collections with collation specifications, you can run the following in the mongo shell:

   db.adminCommand("listDatabases").databases.forEach(function(d){
      let mdb = db.getSiblingDB(d.name);
      mdb.getCollectionInfos( { "options.collation": { $exists: true } } ).forEach(function(c){
         print(mdb[c.name]);
      });
   });

   You can migrate the content of the collection to a new collection without the collation specification (one way is via the aggregation pipeline stage $out).

3. To find indexes with collation specification, you can run the following in the mongo shell:

   db.adminCommand("listDatabases").databases.forEach(function(d){
      let mdb = db.getSiblingDB(d.name);
      mdb.getCollectionInfos().forEach(function(c){
         let currentCollection = mdb.getCollection(c.name);
         currentCollection.getIndexes().forEach(function(i){
            if (i.collation){
               printjson(i);
            }
         });
      });
   });

   Drop the indexes with a collation specification. After the downgrade, recreate the dropped indexes.

4. Convert Data of Type Decimal

1. Connect a mongo shell to the primary.

2. Convert any data of decimal type. In versions of MongoDB earlier than 3.4, operations against documents that contain decimal type may fail. For some possible conversion options, see Model Monetary Data.

   To detect the presence of decimal, you can run db.collection.validate(true) against the collections which may contain decimal data.

   db.collection.validate(true) reports on decimal data only when featureCompatibilityVersion is "3.2".

5. Downgrade Index Versions

If you have v: 2 indexes (i.e. the default version for indexes created in MongoDB 3.4 if featureCompatibilityVersion: "3.4"), reindex the collection to recreate all indexes on the collection as v: 1 before downgrading MongoDB.

1. Connect a mongo shell to the primary.

2. To find indexes with v: 2, you can run the following in the mongo shell:

   db.adminCommand("listDatabases").databases.forEach(function(d){
      let mdb = db.getSiblingDB(d.name);
      mdb.getCollectionInfos().forEach(function(c){
         let currentCollection = mdb.getCollection(c.name);
         currentCollection.getIndexes().forEach(function(i){
            if (i.v === 2){
               printjson(i);
            }
         });
      });
   });

Repeat the process on secondary members of the replica set as the reindex operation does not propagate to the secondaries.