PostgreSQL XML data and Elixir / Ecto?

Questions / Help
,
1

Any well established methods of handling XML data from / to PostgreSQL xml columns using Ecto? How do Elixir veterans do it? If they do, of course… :wink:

2

AFICT, nothing in the ecosystem today (postgrex nor extensions/peers) supports postgres xml types, functions, or queries.

For types, you can use custom Ecto.Types to handle serialization between PG and Elixir. For DDLs, I’d just write my migrations in straight SQL strings, executed by Repo.query! rather than a DSL.

For functions and queries, you’re going to want to use fragments. Essentially, escaping the pure data-modelling of Ecto to insert raw SQL into an otherwise safely Ecto-built query.

This is similar to what I do in my python projects that use XML columns, and it works fairly well. Our use-case is write-once query-occasionally so it’s fairly trivial: we rarely need to read the full document into memory once inserted, and never need to update it.

More integrated functionality would require more work. Perhaps a library calling out to be made?

3

As the efficiency maniac that I will always be, I’d use Erlang’s :xmerl even if I know no XML content would ever be more than 20KB.

But I am not aware of a library that directly adds a custom Ecto.Type for it, no.

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4

Yeah, I was afraid that might be the answer. I looked around a bit but didn’t find anything. I guess the option is to somehow treat the column as text and then do some string parsing, etc. Although even on parsers/builders (especially the latter) choice seems to be limited. Either wrapper on :xmerl (sweetxml was it?) w/o schema support or sax parsers with other limitations. Nothing really full-fledged so far.

5

I’m don’t know how expansive the choice needs to be, really—parsing and building are pretty straightforward until you need SAX, and even then you just use a SAX parser if you’re memory constrained. I’ve built a few XML-handling things in Elixir just fine with sweet_xml and xml_builder. Usually end up with something like this as my main entrypoint, with application-specific helpers for common workflows, and submodules for specific domains (ex XML.SOAP).

XML module 
defmodule XML do
  defmacro __using__(_options \\ []) do
    quote do
      import XML, only: [sigil_x: 1, sigil_x: 2]
    end
  end

  @type document() :: SweetXml.doc()
  @type element() :: SweetXml.xmlElement()

  ####
  # XML builder functions
  ##

  defdelegate document(elements), to: XmlBuilder
  defdelegate document(name, attrs_or_content), to: XmlBuilder
  defdelegate document(name, attrs, content), to: XmlBuilder

  defdelegate element(name), to: XmlBuilder
  defdelegate element(name, attrs), to: XmlBuilder
  defdelegate element(name, attrs, content), to: XmlBuilder

  ####
  # XML parser functions
  ##
  defdelegate generate(any, options \\ []), to: XmlBuilder
  defdelegate parse(document, options \\ []), to: SweetXml
  defdelegate sigil_x(path, modifiers \\ []), to: SweetXml
  defdelegate xpath(parent, spec, subspec \\ []), to: SweetXml
end

That really depends on what you’re doing with the XML. If you’re looking for feedback, share some details about the problem you’re trying to solve with us!

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6

In short the case at hand is:

  • I receive XML document in an xml DB column
  • I need to retrieve it and validate it against XSD schema
  • IF it is valid given the schema then
  • decompose (parse) it into Elixir terms
  • process some data from the parsed document
  • build a response XML document with results of the processing, preferably validate it also
  • store the response document to xml column in the database
  • IF it is not valid then I need only to construct error response XML doc and store it as above.

The columns are of xml type and I cannot change it. I can imagine reading and processing / parsing out data from strings. I can also imagine “shelling out” to xmllint or something to do the validations, etc. But it would be “nice to have” it all in some Elixir lib or so.

P. S. No, it is not SOAP I am doing there :wink:

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7

Yup, in that case I’d make a custom Ecto.Type for serializing/deserializing the xml column between Elixir as binary, and look into erlxml the sdtlib’s xmerl_xsd for XSD validation. You could then consider sweet_xml if you want to use xpath to extract data, or just work with erlxml the stdlib’s xmerl parsing. The xml_builder library should help you build responses and you can pass that back through xmerl_xsd for validation, and a properly implemented Ecto.Type should be capable of serializing resultant binaries back into postgres xml columns.

Truly, you are blessed. :smirking_face:

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8

Roger, thank you. I’ll have to sculpt something out of it. I didn’t say it’s not possible but that nothing I found is “full-fledged” as in “handles all both ways”. Things are scattered, and there are bits and pieces here and there. Even with your suggestions – where you surely have done these type of things before – it’s still three different libs, including an Erlang one. I’d dream of something like e. g. JSON.decode, JSON.encode, as XML.de/encode, augmented with XML.validate and XML Ecto type. Anyway - it is what it is.

Hehe… Yes, I know. Been there, seen that. Not in Elixir though :wink:

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