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Convert an Existing Index to a Unique Index

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  • Before you Begin
  • Steps
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To convert a non-unique index to a unique index, use the collMod command. The collMod command provides options to verify that your indexed field contains unique values before you complete the conversion.

1

Create the apples collection:

db.apples.insertMany( [
{ type: "Delicious", quantity: 12 },
{ type: "Macintosh", quantity: 13 },
{ type: "Delicious", quantity: 13 },
{ type: "Fuji", quantity: 15 },
{ type: "Washington", quantity: 10 }
] )
2

Add a single field index on the type field:

db.apples.createIndex( { type: 1 } )
1

Run collMod on the type field index and set prepareUnique to true:

db.runCommand( {
collMod: "apples",
index: {
keyPattern: { type: 1 },
prepareUnique: true
}
} )

After prepareUnique is set, you cannot insert new documents that duplicate an index key entry. For example, the following insert operation results in an error:

db.apples.insertOne( { type: "Delicious", quantity: 20 } )
MongoServerError: E11000 duplicate key error collection:
test.apples index: type_1 dup key: { type: "Delicious" }
2

To see if there are any documents that violate the unique constraint on the type field, run collMod with unique: true and dryRun: true:

db.runCommand( {
collMod: "apples",
index: {
keyPattern: { type: 1 },
unique: true
},
dryRun: true
} )
MongoServerError: Cannot convert the index to unique. Please resolve conflicting documents before running collMod again.
Violations: [
{
ids: [
ObjectId("660489d24cabd75abebadbd0"),
ObjectId("660489d24cabd75abebadbd2")
]
}
]
3

To complete the conversion, modify the duplicate entries to remove any conflicts. For example:

db.apples.deleteOne(
{ _id: ObjectId("660489d24cabd75abebadbd2") }
)
4

To confirm that the index can be converted, re-run the collMod() command with dryRun: true:

db.runCommand( {
collMod: "apples",
index: {
keyPattern: { type: 1 },
unique: true
},
dryRun: true
} )
{ ok: 1 }
5

To finalize the conversion to a unique index, run the collMod command with unique: true and remove the dryRun flag:

db.runCommand( {
collMod: "apples",
index: {
keyPattern: { type: 1 },
unique: true
}
} )
{ unique_new: true, ok: 1 }

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Unique Indexes

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Index Builds on Populated Collections