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I'm just getting started with MongoDb and I've noticed that I get a lot of duplicate records for entries that I meant to be unique. I would like to know how to use a composite key for my data and I'm looking for information on how to create them. Lastly, I am using Java to access mongo and morphia as my ORM layer so including those in your answers would be awesome.
Morphia: http://code.google.com/p/morphia/
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In database design, a composite key is a candidate key that consists of two or more attributes (table columns) that together uniquely identify an entity occurrence (table row). A compound key is a composite key for which each attribute that makes up the key is a foreign key in its own right.
A compound key is similar to a composite key in that two or more fields are needed to create a unique value. However, a compound key is created when two or more primary keys from different tables are present as foreign keys within an entity. The foreign keys are used together to uniquely identify each record.
There is no conclusion that composite primary keys are bad. The best practice is to have some column or columns that uniquely identify a row. But in some tables a single column is not enough by itself to uniquely identify a row. SQL (and the relational model) allows a composite primary key.
You can use objects for the _id field as well. The _id field is always unique. That way you kind of get a composite primary key:
{ _id : { a : 1, b: 1} } Just be careful when creating these ids that the order of keys (a and b in the example) matters, if you swap them around, it is considered a different object.
The other possibility is to leave _id alone and create a unique compound index.
db.things.ensureIndex({firstname: 1, lastname: 1}, {unique: true}); //Deprecated since version 3.0.0, is now an alias for db.things.createIndex() https://docs.mongodb.org/v3.0/reference/method/db.collection.ensureIndex/
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