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
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.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.Last modified 2yr ago