Flask multiple parameters how to avoid multiple if statements when querying multiple columns of a database from one url

Question

I am trying to build an accounting database using flask as the front end. The main page is the ledger, with nine columns "date" "description" "debit" "credit" "amount" "account" "reference" "journal" and "year", I need to be able to query each and some times two at once, there are over 8000 entries, and growing. My code so far displays all the rows, 200 at a time with pagination, I have read "pep 8" which talks about readable code, I have read this multiple parameters and this multiple parameters and like the idea of using

request.args.get

But I need to display all the rows until I query, I have also looked at this nested ifs and I thought perhaps I could use a function for each query and "If" out side of the view function and then call each in the view function, but I am not sure how to. Or I could have a view function for each query. But I am not sure how that would work, here is my code so far,

@bp.route('/books', methods=['GET', 'POST'])
@bp.route('/books/<int:page_num>', methods=['GET', 'POST'])
@bp.route('/books/<int:page_num>/<int:id>', methods=['GET', 'POST'])
@bp.route('/books/<int:page_num>/<int:id>/<ref>', methods=['GET', 'POST'])
@login_required
def books(page_num, id=None, ref=None):
    if ref is not None:
        books = Book.query.order_by(Book.date.desc()).filter(Book.REF==ref).paginate(per_page=100, page=page_num, error_out=True)
    else:
        books = Book.query.order_by(Book.date.desc()).paginate(per_page=100, page=page_num, error_out=True)
    if id is not None:
        obj = Book.query.get(id) or Book()
        form = AddBookForm(request.form, obj=obj)
        if form.validate_on_submit():
            form.populate_obj(obj)
            db.session.add(obj)
            db.session.commit()
            return redirect(url_for('books.books'))
    else:
        form = AddBookForm()
        if form.validate_on_submit():
            obj = Book(id=form.id.data, date=form.date.data, description=form.description.data, debit=form.debit.data,\
                  credit=form.credit.data, montant=form.montant.data, AUX=form.AUX.data, TP=form.TP.data,\
                      REF=form.REF.data, JN=form.JN.data, PID=form.PID.data, CT=form.CT.data)
            db.session.add(obj)
            db.session.commit()
            return redirect(url_for('books.books', page_num=1))
    return render_template('books/books.html', title='Books', books=books, form=form)

With this code there are no error messages, this is a question asking for advice on how to keep my code as readable and as simple as possible and be able to query nine columns of the database whilst displaying all the rows queried and all the rows when no query is activated All help is greatly appreciated. Paul

I am running this on debian 10 with python 3.7

Edit: I am used to working with Libre Office Base

My question is How do I search one or two columns at a time in My database where I have nine columns out of twelve that I want to be able to search, I want to be able to search one or more at a time, example: column "reference" labels a document reference like "A32", and "account" by a the name of the supplier "FILCUI", possibly both at the same time. I have carried out more research and found that most people advocate a "fulltext" search engine such as "Elastic or Whoosh", But in my case I feel if I search "A32" ( a document number) I will get anything in the model of 12 columns with A 1 2. I have looked at Flask Tutorial 101 search Whoosh all very good tutorials, by excellent people, I thought about trying to use SQLAlchemy as a way, but in the first "Flask Tutorial" he says

but given the fact that SQLAlchemy does not support this functionality,

I thought that this SQLAlchemy-Intergrations will not work either. So therefor is there a way to "search" "query" "filter" multiple different columns of a model with possibly a form for each search without ending up with a "sack of knots" like code impossible to read or test? I would like to stick to SQLAlchemy if possible I need just a little pointer in the right direction or a simple personal opinion that I can test. Warm regards. EDIT:

I have not answered my question but I have advanced, I can query one row at a time and display all the results on the one page, with out a single "if" statement, i think my code is clear and readable (?) I divided each query into its own view function returning to the same main page, each function has its own submitt button. This has enabled me to render the same page. here is my routes code.

@bp.route('/search_aux', methods=['GET', 'POST'])
@login_required
def search_aux():
    page_num = request.args.get('page_num', default = 1, type = int)
    books = Book.query.order_by(Book.date.desc()).paginate(per_page=100, page=page_num, error_out=True)
    add_form = AddBookForm()
    aux_form = SearchAuxForm()
    date_form = SearchDateForm()
    debit_form = SearchDebitForm()
    credit_form = SearchCreditForm()
    montant_form = SearchMontantForm()
    jn_form = SearchJNForm()
    pid_form = SearchPIDForm()
    ref_form = SearchREForm()
    tp_form = SearchTPForm()
    ct_form = SearchCTForm()
    des_form = SearchDescriptionForm()
    if request.method == 'POST':
        aux = aux_form.selectaux.data
        books = Book.query.order_by(Book.date.desc()).filter(Book.AUX == str(aux)).paginate(per_page=100, page=page_num, error_out=True)
    return render_template('books/books.html', books=books, add_form=add_form, aux_form=aux_form, date_form=date_form, debit_form=debit_form,
                       credit_form=credit_form, montant_form=montant_form, jn_form=jn_form, pid_form=pid_form, ref_form=ref_form,
                        tp_form=tp_form, ct_form=ct_form, des_form=des_form)

There is a simple form for each query, it works a treat for each single query. Here is the form and html code:

class SearchAuxForm(FlaskForm):
    selectaux = QuerySelectField('Aux', query_factory=AUX, get_label='id')
    submitaux = SubmitField('submit')
def AUX():
    return Auxilliere.query

html:
   <div class="AUX">
        <form action="{{ url_for('books.search_aux') }}" method="post">
            {{ aux_form.selectaux(class="input") }}{{ aux_form.submitaux(class="submit") }}
        </form>
    </div>

I tried to do this as a single function with one submit button, but it ended in disaster. I have not submitted this as an answer, Because it does not do all I asked but it is a start.

FINAL EDIT:

I would like to thank the person(s) who reopened this question, allowing mr Lucas Scott to provide a fascinating and informative answer to help me and others.

Answer 1

There are many ways to achieve your desired result of being able to query/filter multiple columns in a table. I will give you an example of how I would approach creating an endpoint that will allow you to filter on one column, or multiple columns.

Here is our basic Books model and the /books endpoint as a stub

import flask
from flask_sqlalchemy import SQLAlchemy


app = flask.Flask(__name__)
db = SQLAlchemy(app)  # uses in memory sqlite3 db by default

class Books(db.Model):
    __tablename__ = "book"

    id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    title = db.Column(db.String(255), nullable=False)
    author = db.Column(db.String(255), nullable=False)
    supplier = db.Column(db.String(255))
    published = db.Column(db.Date, nullable=False)

db.create_all()


@app.routes("/books", methods=["GET"])
def all_books():
    pass

The first step is to decide on a method of querying a collection by using url parameters. I will use fact that multiple instances of the same key in a query parameter are given as lists to allow us to filter on multiple columns.

For example /books?filter=id&filter=author will turn into {"filter": ["id", "author"]}.

For our querying syntax we will use comma separated values for the filter value.

example:

books?filter=author,eq,jim&suplier,eq,self published

Which turns into {"filter": ["author,eq,jim", "supplier,eq,self published"]}. Notice the space in self published. flask will handle the url-encoding for us and give back a string with a space instead of %20.

Let's clean this up a bit by adding a Filter class to represent our filter query parameter.

class QueryValidationError(Exception):
    """ We can handle specific exceptions and 
        return a http response with flask """
    pass


class Filter:
    supported_operators = ("eq", "ne", "lt", "gt", "le", "ge")

    def __init__(self, column, operator, value):
        self.column = column
        self.operator = operator
        self.value = value
        self.validate()

    def validate(self):
        if operator not in self.supported_operators:
            # We will deal with catching this later
            raise QueryValidationError(
                f"operator `{operator}` is not one of supported "
                f"operators `{self.supported_operators}`"
            )

Now we will create a function for processing our list of filters into a list of Filter objects.

def create_filters(filters):
    filters_processed = []
    if filters is None:
        # No filters given
        return filters_processed
    elif isinstance(filters, str):
        # if only one filter given
        filter_split = filters.split(",")
        filters_processed.append(
            Filter(*filter_split)
        )
    elif isinstance(filters, list):
        # if more than one filter given
        try:
            filters_processed = [Filter(*_filter.split(",")) for _filter in filters]
        except Exception:
            raise QueryValidationError("Filter query invalid")
    else:
        # Programer error
        raise TypeError(
            f"filters expected to be `str` or list "
            f"but was of type `{type(filters)}`"
        )

    return filters_processed

and now we can add our helper functions to our endpoint.

@app.route("/books", methods=["GET"])
def all_books():
    args = flask.request.args
    filters = create_filters(args.get("filter"))

SQLAlchemy allows us to do filtering by using operator overloading. That is using filter(Book.author == "some value"). The == here does not trigger the default == behaviour. Instead the creator of SQLAlchemy has overloaded this operator and instead it creates the SQL query that checks for equality and adds it to the query. We can leverage this behaviour by using the Pythons operator module. For example:

import operator
from models import Book

authors = Book.query.filter(operator.eq(Book.author, "some author")).all()

This does not seem helpful by it's self, but gets us a step closer to creating a generic and dynamic filtering mechanism. The next important step to making this more dynamic is with the built-in getattr which allows us to look up attributes on a given object using strings. Example:

class Anything:
    def say_hi(self):
        print("hello")

# use getattr to say hello
getattr(Anything, "say_hi")  # returns the function `say_hi`
getattr(Anything, "say_hi")()  # calls the function `say_hi`

We can now tie this all together by creating a generic filtering function:

def filter_query(filters, query, model):
    for _filter in filters:
        # get our operator
        op = getattr(operator, _filter.operator)
        # get the column to filter on
        column = getattr(model, _filter.column)
        # value to filter for
        value = _filter.value

        # build up a query by adding multiple filters
        query = query.filter(op(column, value))

    return query

We can filter any model with our implementation, and not just by one column.

@app.route("/books", methods=["GET"])
def all_books():
    args = flask.request.args
    filters = create_filters(args.get("filter"))
    query = Books.query
    query = filter_query(filters, query, Books)

    result = []
    for book in query.all():
        result.append(dict(
            id=book.id,
            title=book.title,
            author=book.author,
            supplier=book.supplier,
            published=str(book.published)
        ))

    return flask.jsonify(result), 200

Here is everything all together, and including the error handling of validation errors

import flask
import json
import operator
from flask_sqlalchemy import SQLAlchemy


app = flask.Flask(__name__)
db = SQLAlchemy(app)  # uses in memory sqlite3 db by default

class Books(db.Model):
    __tablename__ = "book"

    id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    title = db.Column(db.String(255), nullable=False)
    author = db.Column(db.String(255), nullable=False)
    supplier = db.Column(db.String(255))
    published = db.Column(db.Date, nullable=False)

db.create_all()

class QueryValidationError(Exception):
    pass


class Filter:
    supported_operators = ("eq", "ne", "lt", "gt", "le", "ge")

    def __init__(self, column, operator, value):
        self.column = column
        self.operator = operator
        self.value = value
        self.validate()

    def validate(self):
        if self.operator not in self.supported_operators:
            raise QueryValidationError(
                f"operator `{self.operator}` is not one of supported "
                f"operators `{self.supported_operators}`"
            )


def create_filters(filters):
    filters_processed = []
    if filters is None:
        # No filters given
        return filters_processed
    elif isinstance(filters, str):
        # if only one filter given
        filter_split = filters.split(",")
        filters_processed.append(
            Filter(*filter_split)
        )
    elif isinstance(filters, list):
        # if more than one filter given
        try:
            filters_processed = [Filter(*_filter.split(",")) for _filter in filters]
        except Exception:
            raise QueryValidationError("Filter query invalid")
    else:
        # Programer error
        raise TypeError(
            f"filters expected to be `str` or list "
            f"but was of type `{type(filters)}`"
        )

    return filters_processed


def filter_query(filters, query, model):
    for _filter in filters:
        # get our operator
        op = getattr(operator, _filter.operator)
        # get the column to filter on
        column = getattr(model, _filter.column)
        # value to filter for
        value = _filter.value

        # build up a query by adding multiple filters
        query = query.filter(op(column, value))

    return query


@app.errorhandler(QueryValidationError)
def handle_query_validation_error(err):
    return flask.jsonify(dict(
        errors=[dict(
            title="Invalid filer",
            details=err.msg,
            status="400")
        ]
    )), 400


@app.route("/books", methods=["GET"])
def all_books():
    args = flask.request.args
    filters = create_filters(args.get("filter"))
    query = Books.query
    query = filter_query(filters, query, Books)

    result = []
    for book in query.all():
        result.append(dict(
            id=book.id,
            title=book.title,
            author=book.author,
            supplier=book.supplier,
            published=str(book.published)
        ))

    return flask.jsonify(result), 200

I hope this answers your question, or gives you some ideas on how to tackle your problem.

I would also recommend looking at serialising and marshalling tools like marshmallow-sqlalchemy which will help you simplify turning models into json and back again. It is also helpful for nested object serialisation which can be a pain if you are returning relationships.