expressions.md

Expressions

In addition to the expressions listed in the Ash expressions guide, AshPostgres provides the following expressions

Fragments

Fragments allow you to use arbitrary postgres expressions in your queries. Fragments can often be an escape hatch to allow you to do things that don't have something officially supported with Ash.

Examples

Simple expressions

fragment("? / ?", points, count)

Calling functions

fragment("repeat('hello', 4)")

Using entire queries

fragment("points > (SELECT SUM(points) FROM games WHERE user_id = ? AND id != ?)", user_id, id)

a last resport {: .warning}

Using entire queries as shown above is a last resort, but can sometimes be the best way to accomplish a given task.

In calculations

calculations do
  calculate :lower_name, :string, expr(
    fragment("LOWER(?)", name)
  )
end

In migrations

create table(:managers, primary_key: false) do
  add :id, :uuid, null: false, default: fragment("UUID_GENERATE_V4()"), primary_key: true
end

Like and ILike

These wrap the postgres builtin like and ilike operators.

Please be aware, these match patterns not raw text. Use contains/1 if you want to match text without supporting patterns, i.e % and _ have semantic meaning!

For example:

Ash.Query.filter(User, like(name, "%obo%")) # name contains obo anywhere in the string, case sensitively

Ash.Query.filter(User, ilike(name, "%ObO%")) # name contains ObO anywhere in the string, case insensitively

Trigram similarity

To use this expression, you must have the pg_trgm extension in your repos installed_extensions list.

This calls the similarity function from that extension. See more in the pgtrgm guide

For example:

Ash.Query.filter(User, trigram_similarity(first_name, "fred") > 0.8)