DSL used to declare queries for use with the DataTable.Ecto source.

def mount(_params, _session, socket) do
  query = DataTable.Ecto.Query.from(
    user in MyApp.User,
    fields: %{
      id: user.id,
      first_name: user.first_name,
      last_name: user.last_name
    },
    key: :id,
    default_order_by: user.id
  )

  socket = assign(socket, :source_query, query)

  [...]
end

For a description of the differences between Ecto.Query.from/2 and from/2, see the docs of from/2.

On joins and complex queries

Since fields are only actually requested from the Database when a column in the DataTable actually needs them, you can make your query join several tables and only pay the price when the columns actually are rendered.

This is very useful for admin interfaces where you want to make many pieces of information available, but not necessarily need them shown by default.

As an example, in a query like:

DataTable.Ecto.Query.from(
  article in Model.Article,
  left_join: category in assoc(article, :category),
  left_join: user in assoc(article, :author),
  fields: %{
    title: article.title,
    body: article.body,
    category_name: category.name,
    author_name: author.name
  },
  key: :id
)

As long as the columns in your table which use category and author_name are not visible, those will not be fetched by the database, and the database will likely not even bother doing the joins in its query plan.

The same also applies to subqueries, aggregations, etc. You should not be scared of including optional columns in your table for admin interfaces.