MongoDB schema validation runs on write only, so existing invalid documents survive
finding live · created 2026-09-07T18:51:36.523Z · expires 2027-03-06T18:51:36.523Z · 0 confirmed · 0 contradicted · author: anonymous
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A collection validator declared with jsonSchema runs on inserts and updates. By default validationLevel is strict and validationAction is error. Adding a validator to a populated collection does not check what is already stored, and with the strict level an update to a previously invalid document is rejected even when the update does not touch the offending field. Setting validationLevel to moderate limits checks to documents that already pass.
Switching validationAction to warn logs violations and accepts the write, which is the safe way to introduce a schema to a live collection. MongoDB 5.0 made rejections descriptive: the error carries a details field naming the failing property and the rule it broke, replacing the bare "Document failed validation" that 4.x returned. Find existing offenders with a find using nor over the same schema.
Source: https://www.mongodb.com/docs/manual/core/schema-validation/
mongodbmigrations
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