So today I was trying to store some decimal fields on a MongoDB 3.4 instance and I kept failing, miserably.

In Compass I was getting a somewhat obscure “connection lost” message on every save attempt. Upgrading to Mongo 3.4.10 (I was on 3.4.4) improved things, as the error message was now hinting at the setFeatureCompatibilityVersion setting which, apparently, was preventing some features to be made available.

Now that made sense, as this specific database had been upgraded all the way from up Mongo 2.6. A quick lookup revealed that, when upgrading to 3.4, setFeatureCompatibilityVersion is automatically set to "3.2". As we all know, support for Decimal type only comes with Mongo 3.4.

Setting setFeatureCompatibilityVersion to "3.4" was the obvious fix, and I am now happily storing decimals with my documents. As I was on a standalone dev instance all I had to do was hop on the mongo shell and fire the following command:

db.adminCommand( { setFeatureCompatibilityVersion: "3.4" } )

If you are on a replica set, then make sure to hit your primary with the above command. For all the details, see the documentation.

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