Jonathon Suggs You are right the comparator is somewhat responsible for the "unnecesarry" schema diffs it now creates. However its functionality is completely working IMO. Try creating and comparing your schema example on DBAL level only (without utilizing ORM). You will see that it works. I see and understand that those index name updates are annoying but they are not WRONG. If you have a mapping for a relation with an unnamed index and rename this index in your online schema manually, I would even expect the schema tool to generate this diff. Because this is the whole point of the index rename feature. The problem we have here is that there currently is no way in ORM to define the index name on association mappings. There the following happens in your example when comparing the online and offline schema with the schema tool:
(abbreviated to the important steps, chronological order)
1. Collect mapping information and evaluate offline schema
1.1. Schema tool collects relation SQL for Bar -> Foo association and adds an unnamed index "IDX_76FF8CAA8E48560F" to table "bar"
1.2. Schema tool collects custom indexes defined in the mapping for "bar", detects the custom index "IDX_MANY_TO_ONE", tries to add it to the table "bar, but discards it because there is already another index "IDX_76FF8CAA8E48560F" fulfilling the exact same columns (this is how DBAL works ever since to avoid duplicate indexes)
2. Reverse-engineer online schema from database
2.1 Fetch all defined indexes for table "bar" -> adds "idx_many_to_one" to table "bar"
3. Comapre evaluated online schema to evaluated offline schema
3.1 Compare index "idx_many_to_one" (online schema) to index "IDX_76FF8CAA8E48560F" (offline schema) -> suggest index rename from "idx_many_to_one" to "IDX_76FF8CAA8E48560F" because the mapping is your definition and takes precedence.
Just to clarify what happens under the hood and that it is an ORM issue, not DBAL!