Add Model.exists() to allow efficient checking of at least one matching row

[c-oreills]

Feature Description

Describe the feature you'd like to see implemented

Add a new exists method on model, taking the same parameters as findOne, which returns a boolean of whether at least one matching row exists in the database.

Example usage:

await User.create({ name: 'Alice' });

await User.exists({ name: 'Alice' }); // true
await User.exists({ name: 'Bob' }); // false

Under the hood this would execute a count query limited to 1 and coerce to a boolean, so that it's efficient on the database and network.

Describe why you would like this feature to be added to Sequelize

This is a convenience method. I recently needed this to avoid O(NM) complexity in a check. I ended up using findOne instead and checking for a null, but didn't need the resulting model instance.

The limit of 1 is more efficient for large queries.

The API mirrors a Django's QuerySet.exists

Is this feature dialect-specific?

• No. This feature is relevant to Sequelize as a whole.
• Yes. This feature only applies to the following dialect(s):

Would you be willing to resolve this issue by submitting a Pull Request?

• Yes, I have the time and I know how to start.
• Yes, I have the time but I will need guidance.
• No, I don't have the time, but my company or I are supporting Sequelize through donations on OpenCollective.
• No, I don't have the time, and I understand that I will need to wait until someone from the community or maintainers is interested in implementing my feature.

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