relations

relations

What this shows

Authors write documents; documents have comments and many tags.

• Declaring relations as tagged fields in models/models.go: has_many, belongs_to, many2many with a shared join: table. The generator makes a descriptor per relation (document.Comments), and momo migrate diff adds the foreign keys and the join table.
• momo.Load and momo.LoadAll load relations for a slice of parents in one statement per relation, not one per parent. LoadWhere, LoadOrderBy and LoadPerParentLimit shape the child statement (for example the two newest approved comments of each document); Relation.Scope carries them into LoadAll.
• Select(...).Preload(...) loads as part of the query; Relation.Joined() folds a belongs-to into the parent statement as a LEFT JOIN.
• Has and HasNo filter parents by their children with EXISTS, and CountOrder orders by the number of children. Predicates inside Has name the child as momo_r.
• Link, Unlink, Replace and Count edit a many-to-many link directly.
• CreateGraph inserts a parent and the children you list with Include() in one transaction and copies the generated keys. FindOrCreate is a get-or-insert on a unique key.

Run

cd examples/relations
export DATABASE_URL=postgres://user:pass@localhost:5432/scratch
momo migrate apply
go run .

The test (example_test.go) runs the same steps in a scratch database and checks the output. The full reference is docs/relations.md.