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Diffstat (limited to 'README')
| -rw-r--r-- | README | 27 |
1 files changed, 16 insertions, 11 deletions
@@ -12,33 +12,33 @@ you construct a table relation and convert it to sql: ActiveRelation::Table.new(:users).to_sql -In fact, you will probably never call `#to_sql` directly. Let `users = ActiveRelation::Table.new(:users)`. You can iterate through all rows in the `users` table like this: +In fact, you will probably never call `#to_sql`. Let `users = ActiveRelation::Table.new(:users)`. Rather, you'll work with data from the table directly. You can iterate through all rows in the `users` table like this: users.each { |user| ... } -In other words, Arel relations behave similar regular Ruby collections. Let's have a look at a concrete example: +In other words, Arel relations behave implement Ruby's Eunmerable interface. Let's have a look at a concrete example: users.first # => {'id' => 10, 'name' => 'bob'} -As you can see, Arel converts the rows into a hash, the values of which are the appropriate Ruby primitive (integers, strings, and so forth). +As you can see, Arel converts the rows from the database into a hash, the values of which are sublimated to the appropriate Ruby primitive (integers, strings, and so forth). == Relational Algebra == -Arel is based on the Relational Algebra, a mathematical model that is also the inspiration for relational databases. ActiveRelation::Relation objects do not represent queries per se (i.e., they are not object-representations of `SELECT`, `INSERT`, `UPDATE`, or `DELETE`), rather they represent a collection of data that you can select from, insert into, update, and delete.For example, to insert a row into the users table, do the following: +Arel is based on the Relational Algebra, a mathematical model that is also the inspiration for relational databases. ActiveRelation::Relation objects do not represent queries per se (i.e., they are not object-representations of `SELECT`, `INSERT`, `UPDATE`, or `DELETE` statements), rather they represent a collection of data that you can select from, insert into, update, and delete. For example, to insert a row into the users table, do the following: - users.insert({'id' => 11, 'name' => 'amy'}) + users.insert({users[:name] => 'amy'}) # => INSERT INTO users (users.name) VALUES ('amy') To delete all users: - users.delete + users.delete # => DELETE FROM users To update: - users.update({'name' => 'carl'}) + users.update({users[:name] => 'carl'}) # => UPDATE users SET name = 'carl' -As you can see, the `relation` users does not represent an individual query; rather it is an abstraction on a collection of data and it can produce appropriate queries to do the various CRUD operations. +As you can see, the `relation` named `users` does not represent an individual query; rather it is an abstraction on a collection of data and it can produce appropriate SQL queries to do the various CRUD operations. -=== More Sophisticated Queries === +=== More Sophisticated <strike>Queries</strike> Relations === Following the Relational Algebra, Arel's interface uses some jargon that differs from standard SQL. For example, in order to add a `WHERE` clause to your relations, you use the `select` operation: @@ -47,13 +47,18 @@ Following the Relational Algebra, Arel's interface uses some jargon that differs What would, in SQL, be part of the `SELECT` clause is called here a `projection`: users.project(users[:id]) # => SELECT users.id FROM users + +Joins are fairly straightforward: + + users.join(photos).on(users[:id].equals(photos[:user_id])) => SELECT * FROM users INNER JOIN photos ON users.id = photos.user_id -The best property of the Relational is compositionality, or closure under all operations. For example, to combine the two previous queries: +The best property of the Relational is compositionality, or closure under all operations. For example, to select and project: users \ .select(users[:name].equals('amy')) \ - .project(users[:id]) + .project(users[:id]) \ # => SELECT users.id FROM users WHERE users.name = 'amy' +s == Contributions == |