The collection object

Each model offers an object called ModelName::Collection. This Collection object offers a way to query the database and perform operations on large groups of model.
To instantiate a new Collection, you can simply call the method query over the model class:
users = User.query
Collection and SELECT query
When instantiated, the collection is Crystal implementation of a SELECT query:
-- Users.query ==
SELECT * FROM users
Therefore, calling to_a over the Collection will fetch all the models from the database to your crystal code:
array_of_users = User.query.to_a
Collection will really perform SQL request only on resolution time, when calling each or to_a for example.
Mutability
Collection are mutable objects, and many of the methods will change the state of the collection:
query = User.query # SELECT * FROM users;
query.select("id") # SELECT id FROM users;
query.select("id") # SELECT id, id FROM users;
Therefore, you may want to use Collection#dup to duplicate the current state of the collection:
query = User.query # SELECT * FROM users;
query.select("id") # SELECT id FROM users;
​
query2 = collection.dup # SELECT id FROM users;
Collection can be filtered and refined to query exactly what you want. Actually, they are refined version of SQL::SelectBuilder object described in Low-level SQL chapter.

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Locks

Filter the query

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Collection and SELECT query

Mutability