Sure, so let’s do some practice. What I don’t recommend is trying to look at the ecto functions and see how they work, because they’re pretty complicated internally. Technically they’re all macros, so it’s just a bit of a pain to figure out what is going on. There’s a reason for this (compile time protection from sql injection, yay!) but it’s definitely complicated.
Fortunatelly, using the ecto query macros isn’t that bad, but there’s no substitute for practice. I highly recommend working through the programming ecto book, or any of the other Elixir books that uses ecto (programming phoenix, the graphql book, etc). If you want to go for it just based on docs that’s fine too, just build some simple exercises for yourself. Here’s an example of a function from the graphql book that takes this kind of input:
%{filter: %{category: "drinks", priced_below: 10.00}, order: :desc}
And then builds an ecto query that filters / orders an Item schema accordingly:
def list_items(args) do
args
|> items_query
|> Repo.all
end
def items_query(args) do
Enum.reduce(args, Item, fn
{:order, order}, query ->
query |> order_by({^order, :name})
{:filter, filter}, query ->
query |> filter_with(filter)
end)
end
defp filter_with(query, filter) do
Enum.reduce(filter, query, fn
{:name, name}, query ->
from q in query, where: ilike(q.name, ^"%#{name}%")
{:priced_above, price}, query ->
from q in query, where: q.price >= ^price
{:priced_below, price}, query ->
from q in query, where: q.price <= ^price
{:added_after, date}, query ->
from q in query, where: q.added_on >= ^date
{:added_before, date}, query ->
from q in query, where: q.added_on <= ^date
{:category, category_name}, query ->
from q in query,
join: c in assoc(q, :category),
where: ilike(c.name, ^"%#{category_name}%")
{:tag, tag_name}, query ->
from q in query,
join: t in assoc(q, :tags),
where: ilike(t.name, ^"%#{tag_name}%")
end)
end
If we pass the example input into the items_query function we see that it returns an ecto query:
iex(2)> query = PlateSlate.Menu.items_query(%{filter: %{category: "drinks", priced_below: 10.00}, order: :desc})
#Ecto.Query<from i in PlateSlate.Menu.Item, join: c in assoc(i, :category),
where: ilike(c.name, ^"%drinks%"), where: i.price <= ^10.0,
order_by: [desc: i.name]>
This query can then be executed by our repo:
Repo.all(query)
I’m suggesting that you should build a function similar to my items_query function that recursively walks through your yecc output and matches on various sub parts, reducing on to the ecto query data structure.
