8. Create a MarketVendor

[JorjaF]

Details
This endpoint should follow the pattern of POST /api/v0/market_vendors, and it should create a new association between a market and a vendor (so then, the vendor has a new market that they sell at).

When valid ids for vendor and market are passed in, a MarketVendor will be created, and a response will be sent back with a 201 status, detailing that a Vendor was added to a Market.

After implementing the happy path for this endpoint, run it, and check that when you call GET /api/v0/markets/:id/vendors for the vendor in which you just added to a market, that you see the newly associated market listed.

If an invalid vendor id or and invalid market id is passed in, a 404 status code as well as a descriptive message should be sent back with the response.

If a vendor id and/or a market id are not passed in, a 400 status code as well as a descriptive message should be sent back with the response.

If there already exists a MarketVendor with that market_id and that vendor_id, a response with a 422 status code and a message informing the client that that association already exists, should be sent back. Looking at custom validation might help to implement a validation for uniqueness of the attributes for this resource.

Example #1 😁
Request:

POST /api/v0/market_vendors
Content-Type: application/json
Accept: application/json
Body:

{
"market_id": 322474,
"vendor_id": 54861
}
(where 322474 and 54861 are valid market and vendor id's.)
Response: status: 201

{
"message": "Successfully added vendor to market"
}

Example #2 😭
Request:

 POST /api/v0/market_vendors
 Content-Type: application/json
 Accept: application/json

Body:

{
"market_id": 987654321,
"vendor_id": 54861
}
(where 987654321 is an invalid market id)
Response: status: 404

{
"errors": [
{
"detail": "Validation failed: Market must exist"
}
]
}

Example #3 😭
Request:

 POST /api/v0/market_vendors
 Content-Type: application/json
 Accept: application/json

Body:

{
"market_id": 322474,
"vendor_id": 54861
}
(where 322474 and 54861 are valid market and vendor id's, but an existing MarketVendor with those values already exists.)
Response: status: 422

{
"errors": [
{
"detail": "Validation failed: Market vendor asociation between market with market_id=70 and vendor_id=1150 already exists"
}
]
}