"select form" does not sync with model - angularjs

Question

This is my template:
<div class="form-group"
ng-repeat="recipeIngredient in recipes.currentRecipe._ingredients">
<label class="col-sm-2 control-label">Ingredient {{ $index + 1 }}</label>
<div class="col-sm-10 form-inline" role="form">
<input
type="text"
class="form-control"
placeholder="quantity"
ng-model="recipeIngredient.quantity"
>
<input
type="text"
class="form-control"
ng-model="recipeIngredient._unit"
>
<select
class="form-control"
ng-model="recipeIngredient._unit"
ng-options="unit._id for unit in units"
/>
<select
ng-model="recipeIngredient._ingredient"
class="form-control"
ng-model="recipeIngredient._ingredient"
ng-options="ingredient._id for ingredient in ingredients"
/>
<button
type="button"
class="btn btn-default btn-xs glyphicon glyphicon-remove"
ng-click="recipes.currentRecipe.removeRecipeIngredient($index)">
</button>
</div>
</div>
I want to display the ingredients listed in the currentRecipe
If I use:
<input type="text" class="form-control" ng-model="recipeIngredient._unit">
I can see the value of _unit showed
But why the select form does not show at all the current value of recipeIngredient._unit?
<select
class="form-control"
ng-model="recipeIngredient._unit"
ng-options="unit._id for unit in units"
/>
I've verified the the _unit is part of the list of units
If I change the value of the select box to some value, the input text show "Object", which means the real value of the model is the unit object and not the unit._id text
ng-options="unit._id for unit in units"`
Please tell me if you want additional code

Answer 1

I've found out that when I use:
ng-options="unit._id for unit in units"
The text showed in the drop box is unit._id, but the ng-model will be the object in unit and not the text showed!
To correct my problem I've written like this:
<select ng-model="recipeIngredient._unit" class="form-control">
<option ng-repeat="unit in getId(units, '_id')">{{unit}}</option>
</select>
the getId is the function that return a list of _id from units