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.